Synthesis and physico-chemical properties of new 3-benzyl-4-thioxo-5-arylideneimidazolidine-2-ones and 3-benzyl-5-arylideneimidazolidine-2,4-dione are described. These compounds were synthesized by condensation reaction from aromatic aldehydes and 3-substituted imidazolidine-2,4-diones or 4-thioxoimidazolidine-2-ones. The N-alkylation of 5-benzylideneimidazolidine-2,4-dione led simultaneously to mono- and dialkylated derivatives. The nucleophilic addition of 1-methyl-3-benzylimidazolidine-2,4-dione with 2-cyano-3-(3,4-dichlorophenyl) acrylate also yielded the 3-substituted 5-arylideneimidazolidine-2,4-dione derivative. Antimicrobial in vitro activity was determined on some compounds.