A quasi-white photoluminescence (PL) emission of colloidal CdS nanocrystals (NCs) of various sizes (2, 3, and 6 nm) embedded in a polymer matrix was examined by optically detected magnetic resonance (ODMR) spectroscopy. The registered ODMR spectra are complex, composed of both positive and negative signals. The positive component of the ODMR signal depends strongly on both particle size and the range of the PL wavelength registration. Based on this behavior it is ascribed to the complex paramagnetic centers formed by electron-and hole-trapping species coupled by an exchange interaction. By variation of the state of the NC/polymer interface, it is shown that the electron-trapping center is located at the interface. The sharp negative component of the ODMR spectra does not depend on the NC size. It was assigned to a sulfur vacancy (V-s) acting as a center of nonradiative recombination.