A series of new 3,5-diaryl-N-hydroxymethylene-1,4-thiazine-1,1-dioxides and 3,5-diaryl-N-methoxymethylene-tetrahydro-1,4-thiazines-1,1-dioxides were synthesized and their antimicrobial activities were tested against Gram-positive (Bacillus subtilis, Streptococci) and Gram-negative bacteria (Escherichia coli and Klebsiella penumoniae) and against the fungi Penicillium inclobium and Aspergillus flavus. The synthesized compounds have been characterized on the basis of elemental analyses, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR).