In light of the pronounced heterogeneous reactivity of molybdenum, tungsten, and vanadium oxides toward hydrocarbons, 1 it is somewhat surprising that no polyoxoanion clusters of these metals are known which contain hydrocarbon moieties bound to their surface oxygens. A recent study of the cobalt molybdate catalyzed selective oxidation of propylene to acrolein has provided evidence indicating that the rate-determining step is desorption of acrolein from the oxide catalyst surface. As a logical starting point for the structural investigation of polyoxoanion-hydrocarbon interactions, we have therefore examined the solution reactivity of acrolein and other aldehydes toward polyoxomolybdates. We report here the isolation and characterization of several presumably isostructural RCHMo4O15H3- anions (R = C2H3, H, CH3, C6H5, and CF3), which contain RCH units bonded to the oxide surface of a tetranuclear polyoxomolybdate cluster. © 1979, American Chemical Society. All rights reserved.