The crystal and molecular structures of 3-methyl-2,4-diphenyl-(1,3)-thiazolidine-5-spiro-2′-adamantane and 3-methyl-2,4,5,5-tetraphenyl-(1,3)-thiazolidine are investigated showing the existence of C(sp 2 )-H‚‚‚S and C(sp 2 )-H‚‚‚N intramolecular contacts. The use of the Bader theory shows that C-H...S interactions existing in crystal structures may be treated as weak H bonds. The C-H...N and C-H...S interactions are also analyzed here for simple modeled complexes of (1,3)-thiazolidine as the proton acceptor and simple proton donators: HF, H 2 O, C 2 H 4 , and C 2 H 2 molecules. The calculations for these complexes were performed within the DFT method, B3LYP/6-311++G** level of theory. The bond critical points (BCPs) were found for these modeled systems and the analysis of the electron densities and their Laplacians at BCPs was performed.