Disubstituted [1,3,4]-oxadaiazoles (6a-b), Mannich bases (7a-f) and S-alkylated derivatives (8a-f) have been synthesized from 4-(methylsulfonyl/sulfinyl)phenylaceticacid (3a-b) through a multi-step reaction sequence starting from (4-methylthio)phenylacetonitrile (1). The structures of new compounds were established on the basis of their elemental analysis, IR, 1H NMR, 13C NMR and mass spectral data. All the synthesized compounds were screened for their antioxidant, antibacterial and antifungal activity. Some of the derivatives have promising antimicrobial activity.