Three compounds were found to have been illegally added to dietary supplements marketed for erectile dysfunction. The structures of these compounds were identified with NMR, HRMS, IR, UV and LC/MS/MS, after comparison with the structure of sildenafil, data showed that the oxygen atom of the carbonyl group in the heterocyclic ring had been replaced with a sulfur atom, these thio-sildenafil analogues were identified as thiosildenafil, thiohomosildenafil and hydroxythiohomosildenafil. Since their structures were similar to that of sildenafil, according to structure-activity relationship, the side effects of these analogues might also associate with that of sildenafil. In accordance with this finding, thio-sildenafil analogues have been included in the inspection list of illegal adulterants in Taiwan. From January to October 2009, thiosildenafil was detected in 6 samples, thiohomosildenafil was detected in 4 samples and hydroxythiohomosildenafil was detected in 3 samples.