To investigate the release profile of the aspirin-cysteine conjugate, we developed a modified FeCl3 assay that selectively reacts with aspirin in simulated intestinal fluid. Briefly, we prepared the conjugate by the EDC coupling method and characterized it by NMR spectroscopy. Then, we added 2 mL of simulated intestinal fluid (SIF, pH 7.4) to a quartz cuvette and monitored the absorbance at 510 nm using a UV-Vis spectrophotometer. After recording the baseline for 1 min, we added a known amount of the conjugate to the cuvette and immediately started the timer. We then added 200 µL of 0.1 M FeCl3 solution to the cuvette at 5 minute intervals, which allowed FeCl3 to selectively react with aspirin in the conjugate and form a colored complex that absorbs at 510 nm. We confirmed the specificity of the assay by testing the reaction of FeCl3 with ascorbic acid, which does not interfere with the conjugate. We also validated the assay by comparing the release profile of the conjugate to that of the parent drug aspirin in SGF using a similar protocol. The release kinetics of the conjugate were analyzed by fitting the absorbance data to first-order kinetics.