Several bismuth(III) carboxylates (formate, acetate, propionate, oxalate, lactate) and the analogous tervalent lanthanum, samarium and praseodymium formates and acetates were synthesized and characterized in view of their use as precursors for the formation of bismuth- or lanthanide-based molybdate catalysts upon controlled thermal decomposition on MoO3 support. Mixed Bi-Ln carboxylates were prepared and used in parallel as precursors for Ln-promoted bismuth molybdate catalysts. Both the nature of the lanthanide element and the calcination temperature were found to determine the formation of specific molybdate-type phases on the support. Detailed examination of these catalysts by X-ray diffraction and FTIR spectroscopy identified alpha-M(2)Mo(3)O(12) phases in Bi- and Pr-containing samples calcined at 823 K, and hexamolybdate phases M(2)Mo(6)O(21) in La- or Sm-containing catalysts calcined at 973 K.