Colloidal uncapped and starch capped CdS (SCdS) nanoparticles were prepared and interaction with bovine serum albumin (BSA) have been studied by UV-visible, FT-IR, steady state, time resolved and synchronous fluorescence spectroscopic measurements. BSA molecules adsorbed on the surface of colloidal CdS through the capping agent. The apparent association constant (K(app)=2.54x10(2) M(-1)) and degree of association has been calculated (alpha=1.12) from absorption studies. The binding constant from fluorescence quenching method (6.6x10(2) M(-1)) matches well with that determined from the absorption spectral changes. Static quenching mechanism and conformational changes on BSA molecules were confirmed by time resolved and synchronous fluorescence measurements respectively. The effect of starch capped CdS on the conformation of BSA has been analyzed by means of UV-visible absorption and synchronous fluorescence spectra.