A high-performance liquid chromatography (HPLC) technique has been developed for determining foreign impurities in tropoxin parent substance and tablets. Reliable separation of tropoxin and impurities was achieved in gradient elution mode on a liquid chromatograph equipped with a spectrophotometric detector and a stainless steel column (250 × 4.6 mm) packed with C18 (5 μm) sorbent. The detection limits for tropoxin and identified impurities were determined. It was found that the area under the impurity peak was a linear function of the tropoxin concentration in stock solutions in the range from 0.02% to 2%. The results were repeatedly reproduced in model mixtures, parent substance samples, and different batches of tropoxin tablets.