CORRECTING and REPLACING Sigma-Aldrich Plant Damaged in Explosion, Resulting in One Minor Injury and Precautionary Evacuation in Miamisburg, Ohio.

Business Editors/Assignment Desks/Health/Medical Writers

CORRECTION...by Sigma-Aldrich

BIOWIRE2K

ST. LOUIS--(BUSINESS WIRE)--Sept. 22, 2003

In BW5754 issued Sept. 22, 2003: In the first graph, second sentence of the release the compound involved in the explosion should be nitric oxide (sted nitrous oxide).

The corrected release reads:

SIGMA-ALDRICH PLANT DAMAGED IN EXPLOSION, RESULTING IN ONE MINOR INJURY AND PRECAUTIONARY EVACUATION IN MIAMISBURG, OHIO 

An explosion at the Sigma-Aldrich (Nasdaq:SIAL) facility in Miamisburg, Ohio injured one employee and resulted in a precautionary evacuation of those living within one mile of the plant. On Sunday morning, September 21, 2003, a tank containing nitric oxide exploded as Company employees and Miamisburg City and Township firefighters and a hazardous material team were checking a leak at the plant. The facility containing the tank was largely destroyed.

The injured employee suffered a cut on the head and was treated at a local hospital and released. About 2,000 residents in the surrounding area were immediately evacuated pending a review of the incident. Local emergency officials have allowed Company employees access to the site to assess damage and permitted residents to return to their homes this morning.

At the Miamisburg site, the Company's 75 employees produce stable isotopes and isotopically labeled compounds used in life science research, medical diagnostics and PET imaging. Other facilities on the site were damaged, but are expected to resume production after a thorough safety review and minor repairs are completed. The cause of the explosion is being investigated by both Company and government officials. Products manufactured at this facility contribute less than two percent of Sigma-Aldrich consolidated revenues. Insurance coverage is expected to cover the majority of property damage and business interruption costs. The Company has established a toll-free number (800-811-5352) for area residents to call with questions or concerns. Others are asked to contact the Company as indicated below.

In commenting on the incident, Michael R. Hogan, Chief Administrative Officer and CFO said: "At times like these, it's important to focus on what matters most. We're thankful that only one person - a Company employee on site to investigate a leak - was injured. And we're even more thankful that that injury was not a major one."

"We also appreciate and thank our neighbors in Miamisburg who have shown remarkable patience throughout the disruption this has caused in their daily lives. Local emergency personnel from the township and city fire departments, the Red Cross, local school officials and others all responded quickly and professionally to our calls for help and have been invaluable in maintaining calm, restoring a safe environment at our facility and thereby permitting area residents to return to their homes."

About Sigma-Aldrich: Sigma-Aldrich is a leading Life Science and High Technology company. Our biochemical and organic chemical products and kits are used in scientific and genomic research, biotechnology, pharmaceutical development, the diagnosis of disease and chemical manufacturing. We have customers in life science companies, university and government institutions, hospitals and in industry. Over one million scientists and technologists use our products. Sigma-Aldrich operates in 34 countries and has 6,000 employees providing excellent service worldwide. We are committed to the success of our Customers, Employees and Shareholders through leadership in Life Science, High Technology and Service. For more information about Sigma-Aldrich, please visit our award-winning web site at www.sigma-aldrich.com

Cautionary Statement: This release contains forward-looking statements relating to future performance and actions and other statements regarding the Company's expectations, beliefs, intentions and the like regarding future sales, earnings and other matters. These statements involve assumptions regarding Company operations and conditions in the markets the Company serves. Although the Company believes its expectations are based on reasonable assumptions, such statements are subject to risks and uncertainties, including, among others, certain economic, political and technological factors. The Company does not undertake any obligation to update these forward-looking statements.