PARED DOWN PARADIGM.

Alvaro Siza and Eduardo Souto de Moura have cooperated to make one of the simplest and most powerful of all Expo's pavilions. It combines profound knowledge of Portuguese materials with a lyrical approach to technology.

The Portuguese pavilion brilliantly sums up what the rest of the world has come to expect of the best of the nation's architects, not surprisingly perhaps, for it has been produced by two of them, who, judging by the clarity and precision of the result, must surely have worked harmoniously together.

In plan, it is a square with a long finger and a short thumb sticking out south-west towards the Europa Boulevard, the main avenue of the east side of the exhibition grounds. Two storeys high, the front wall, the one at the end of the finger nearest the avenue, is made of smooth, slightly rosy Portugese limestone with PORTUGAL plainly and lightly incised in gold over a single central opening. To the left of this most understated of advertisements is the entrance canopy, a very simple steel structure carrying underneath a plane of beautifully smoothed planks; it is made strong enough to sail, almost supportless, along the front on the boulevard and turn towards the main public entrance. In doing so, it encloses a small court where a couple of cork trees grow in chaste elegance. The two sides of the court defined by the building itself are clad in small square glossy hand-made tiles; Portuguese ceramics have changed a lot since the seventeenth century, but they can still be very powerful. To the left, the wall with the entrance door is a deep golden yellow, the colour of the sun and welcome. Straight ahead is a rich blue plane, signalling the end of the approach route and the prospect of calm within.

Inside, you are inflected by the Aalto-like curve of the pale stone reception desk out from the comparatively cramped space under the upper floor into the double-height square of the main exhibition area. This is quite astonishing. A completely irrational white translucent roof undulates over the space like a gentle cloud, or the sky seen from under water on a pellucid day. It is gentle, but utterly different from the calm rigorous geometry of the approach. (Though there were slight hints that all was not quite so orthodoxly Rational as appears at first sight by the slight inclination of the canopy from the rigorous grid of the main building.)

The roof, evolved with Ove Arup & Partners, is a deformed space-frame, fabricated as has been possible for only a few years, with every node and every strut slightly different to make the curves and billows. It was assembled from its precisely prefabricated components on the ground, then hoisted up to bear on steel staunchions incorporated in the walls. External roof cladding is pvc coated polyester fabric; the internal skin is made of the purest white cotton. Calm, pale daylight suffuses the whole space, complemented when necessary with lamps in the cavity between the skins (the system also incorporates a degree of heating to cope with the unlikely event of snowfall).

Apart from the big space, the pavilion is a set of quite conventional but pleasant rooms, with a meeting area (or VIP entrance) approached through the opening in the limestone front wall, and a shop on the ground floor. The first floor has a little auditorium, offices with a meeting room and lavatories. All seems almost boringly utilitarian until you get breathtaking glimpses down and across the big magic volume.

Emerging, blinking, into the sun and looking about, it quickly becomes clear that the little building is much more complicated than it seemed when you went in. The main bulk is clad in compressed cork blocks apparently assembled like masonry, and waterproofed with a compound that colours them a strong brown-black. In fact, the cork walls are prefabricated panels which, like the modular steel structure, will be demounted at the end of Expo to be re-erected in Portugal. As you walk round the comforting, quiet warm dark-brown box, you become aware of the strangeness of the whole thing, for the undulations of the roof are echoed in the curves of the walls against the sky.

The Portuguese pavilion is perhaps an argument for expos. Here is a small and rather poor country making an exquisite, imaginative, economical thing, and showing up the stupidity of the blundering bureaucratic taste of big rich nations like the Germans and French (not to speak of the appalling market mediocrity of the British). It simultaneously demonstrates deep traditional appreciation of materials, light and space, and a lively understanding of the potentials of modern technology, without swanking about them. In its modest gentle way, it is a triumph.