The general principles of a new concept of chemical analysis, called stoichiography, are represented. Stoichiography allows the reference-free analyzing of the molecular and phase composition of complex substances. The differential dissolving technique as a new stoichiography method was developed to characterize in detail the phase composition of heterogeneous catalysts and to identify known and absolutely unknown phases composing the catalyst. The analyzed phases may be in crystalline or amorphous state, which is of no importance for the analysis performance. Such phase analysis is performed with a high sensitivity and very fast. Equipment, developed specially for this analysis, is called stoichiograph. The advantages of differential dissolving in studying the phase composition of catalysts are demonstrated through many particular examples.