In Cd(S,Se)‐doped glasses the optical properties are strongly dependent on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on both stoichiometry and size is difficult to obtain in crystals smaller than 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation of amplitude of this intensity with the energy of the x ray—‘‘the anomalous effect’’—near the selenium edge is related to stoichiometry. Anomalous small‐angle x‐ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lower than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540–650 °C. The samples treated at temperatures above 580 °C contain crystals with size larger than 4 nm. For all these samples the anomalous effect has nearly the same amplitude, and we found the stoichiometryx=0.4 for the CdSxSe1−xnanocrystals. This agrees with the previous results obtained by scanning electron microscopy and Raman spectroscopy. The results are also confirmed by measurements of the position of the optical absorption edge and by wide‐angle x‐ray scattering experiments. For the sample treated at 560 °C, the nanocrystal size is 3 nm and the stoichiometryx=0.6 is deduced from the anomalous effect. For samples treated at lower temperatures the anomalous effect is not observable, indicating an even lower selenium content in the nanocrystals (x≳0.7). We observed differences in the Se content of nanocrystals for different heat treatments of the same initial glass. These results may be very helpful to interpret the change in the optical properties when the temperature of the treatments decreases in the range 560–590 °C. In this temperature range, compositional effects seem to be of the same order of magnitude as the effects of the quantum confinement. ©1995 American Institute of Physics.