In this work we report on peculiarities of technique for correct measurements of optical transmittance, optical density and Faraday rotation spectra of colloidal semiconductor nanoparticles and nanocomposites. As an example we have used CdS:Mn nanoparticles embedded in polyvinylalcohol (PVA) matrix. With the decrease of the size of CdS:Mn nanoparticles, the absorption exciton band was shifted to shorter wavelengths as a result of quantum confinement effect. In order to observe clearly this shift we have used several samples with the same average nanoparticle size, but with different sample thicknesses. It was made analysis of the obtained data on optical spectra for nanocomposites with comparison for bulk semiconductor materials. Similar methodology was applied for magneto-optical spectra of colloidal solutions and nanocomposites.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.