Background: The chemical stability of a pharmaceutical product depends, among other factors, on environmental factors during transport, storage and manipulation of the product. Aim: To study the chemical stability of acetylsalicylic acid (AAS) tablets during ten months of storage in five pharmacies of Concepción, Chile. Material and methods: Tablets were randomly collected at the beginning of the study and at the third, sixth and tenth month. Quantitative analyses of AAS tablets was carried out by instrumental thin layer chromatography (HPTLC). Results: AAS in tablets was between 99 and 109% at the beginning of the study, between 76 and 110% at three months, between 71% and 112% at six months and between 86 and 110% at ten months of storage. Conclusions: There was a progressive decrease in the content of acetylsalicylic acid in tables during storage, but it remained between the limits accepted by the United States Pharmacopoeia (USP) (90-110%) (Rev Méd Chile 2002; 130: 409-415)