A quantitative EPR study of blue ultramarine pigments has been performed in order to determine the concentration of the S-3(-) chromophore. Copper sulfate CuSO4.5H(2)O has been used as a standard, while a ruby crystal was used as an inner standard to take into account the changes of the quality factor of the cavity. These experiments show that, in the most-colored pigments, less than half of the socialite cages are occupied by a S-3(-) radical. In other experiments, it has been shown that the blue ultramarine pigments can be significantly modified by heating under a dynamic vacuum. The concentrations of S-3(-) and S-2(-), as deduced from EPR and Raman experiments, are increased after this type of treatment. These changes imply that sulfur species are transformed into S-3(-) or S-2(-) during this treatment. It is discussed that these sulfur species could be S-2(-).