Abstract: To investigate the characteristics of mass spectra of four sildenafil piperidine derivatives, their fragments were analyzed by electrospray ion trap mass spectrometry in a multi-stage tandem MS full scan mode performed on a Finnigan LCQ instrument. The mass spectrometry was operated in the negative mode. The mass spectra were obtained mainly by analyzing the fragments of the four compounds, the structures of which resemble with each other except the substituted alkyl group. The full scan MS 2 spectra of each compound gave characteristic fragment ions formed through cleavage of the sulfony amide S-N bond; The full scan MS 3 spectra gave characteristic fragment ions obtained through the sulfony C-O cleavage after elimination of sulfur dioxide; The full scan MS 4 spectra gave characteristic fragment ions formed by loss of the alkyl group on the alkoxy substituent on the phenyl ring as alkyl radical. These characteristics can be used for future structure elucidation in studies of analogues of sildenafil.