The luminescence behavior of CdS nanocrystals in aqueous solution and in the presence of proteins has been deeply investigated. CdS nanocrystals have been prepared in water by thermal decomposition of a single organometallic precursor assisted by thioglycerol, which acts as capping agent. Different experimental conditions have been explored to gain insights into the parameters affecting the nanocrystal growth. The CdS samples were characterized in terms of absorption and emission spectra, luminescence quantum yields, and decay times. These data together with size distribution analysis gave information on the growth mechanism and on the nature of the trap states formed in different experimental conditions, The emission properties of the nanocrystals in the presence of bovine serum albumin (BSA) have been examined to test how the electrostatic bioconjugation can influence the optical properties of the nanocrystals. The spectral changes observed upon addition of BSA indicated a direct interaction of the protein with the nanocrystal surface able to recover (at least partially) the defects formed during the crystal growth, resulting in improved emission properties.