New complexes of copper(II) with S-alkenyl derivatives of thiosalicylic acid (alkenyl = propenyl-(L1), isobutenyl-(L2)) have been synthesized and characterized by microanalysis, infrared spectra, magnetic measurements, and by NMR spectra. The cytotoxic activity of two newly synthesized precursor S-alkenyl derivatives of thiosalicylic acid were tested using an MTT colorimetric technique on HCT-116 human colon carcinoma cells. The cytotoxic effect of the copper(II)- complexes were higher compared to the cytotoxicity of the corresponding ligand (for concentrations from 31.25 to 250 μM). Copper(II)-complexes showed a slightly lower cytotoxicity compared to cisplatin. Complexes of copper(II) with S-alkenyl derivatives of thiosalicylic acid (at concentrations from 250 to 1000 μM) had a cytotoxic effect on HCT-116 cells compared to cisplatin.