Abstract The antithrombotic effect of aspirin has been demonstrated through the use of small daily doses of the drug. Slow release formulations were shown to be adequate dosage form for this purpose. In this study, two sustained release formulations of aspirin were prepared and compared to plain aspirin tablets for their drug release and behavior upon storage at 40°C/90% relative humidity (R.H.). The polyacrylate-polymethacrylate based polymers were found suitable for the preparation of a sustained release aspirin product. The release characteristics of the drug were adequate to meet its antithrombotic effect. Storage-related changes observed in this study should help in better understanding and proper treatment of such formulations for the sake of obtaining more stable products with predictable uniform drug release.