[en] Raman spectra of phases obtained by cooling molten mixtures of Na₂Mo₂O₇ and Na₂W₂O₇ are in agreement with the expectations for the formation of a substitutional phase Na₂Mosub(2-x)Wsub(x)O₇ with statistical distribution of Mo and W. The assignment of observed Raman bands to vibrations of characteristic groups in the polyanions is confirmed by the frequency shifts from pure Na₂Mo₂O₇ to pure Na₂W₂O₇. K₂Mo₂O₇ with an anionic structure completely different from that of K₂W₂O₇ also allows a statistic substitution of Mo by W without structural change. The range of homogeneity extends till 66 mol% W. The formation of metastable K₂Mo₂O₇-II is favoured by the introduction of W for Mo. Ag₂Mo₂O₇ and Ag₂W₂O₇ also have different polyanionic structures. Their Raman spectra are given and discussed partly. The Ag₂Mo₂O₇ structure allows a moderate substitution of Mo by W. A substitution of Mo for W in the Ag₂W₂O₇ structure could not be detected. (author)