The La 2 W 2Àx Mo x O 9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La 2 W 2 O 9 has two polymorphs and the low-temperature phase (a) transforms to the high-temperature form (b) at 10771C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The b structure for xZ0:7 compositions can be prepared as single phase at any cooling rate. The b phase for 0:3rxr0:7 compounds can be prepared as single phase by quenching, whereas a mixture of a and b phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the b-phase having a higher Mo content. The x ¼ 0:1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of b-La 2 W 1.7 Mo 0.3 O 9 has been analyzed by the Rietveld method in P2 1 3 space group. The final R-factors were R WP ¼ 9:0% and R F ¼ 5:6% with a structure similar to that of b-La 2 Mo 2 O 9 . Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9 Â 10 À6 and 9.7 Â 10 À6 1C À1 for a