Addition of H2S and H2Se, in different proportions, to aqueous Cd(ClO4)(2) solutions containing (NaPO3)(6) led to the formation of coupled composite CdS-CdSe nanoparticles. Conversely, the sequential introduction of different amounts of H2S and H2Se (or H2Se and H2S) into aqueous Cd(ClO4)(2) solutions containing (NaPO3)(6) produced core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles. These structural assignments were based on spectroscopic (absorption, transmission, excitation, and emission), electrochemical, pulse radiolytic, and electron microscopic measurements. Conditions were adjusted (by adding excess cadmium and hydroxide ions) for the maximization of excitonic fluorescence. Two emission bands were observed in the coupled and in the core-shell type mixed semiconductor nanoparticles. The first one, centered around 470 nm, was attributed to the 1s(e)-1s(h) excitonic emission of CdS. The second, centered around 560 nm, was proposed to arise from charge transfer of CdS to the coupled composite CdS-CdSe and core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles.