CuInS2 nanoparticles have been synthesized via solvent thermolysis of novel bimetallic complexes of the general formula [(Ph3P)2CuIn(S2COR)4] (where R = CH3; C2H5; C(CH3)2); and [(Ph3P)2CuIn(SCH2CH2S)2]. These complexes have been prepared by the reactions of Na/KS2COR and NaSCH2CH2SNa with InCl3 and [(Ph3P)2CuNO3] in methanol, respectively. Solvent thermolyses of these complexes were carried out in ethylene glycol at 196 degrees C for different time periods. The nanoparticles obtained were characterized extensively by techniques like powder X-ray diffraction, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy. The optical band gap of the nanoparticles was determined by diffuse reflectance spectroscopy (DRS).