The interaction of [Os₃(CO)₁₂] on silica, alumina, and titania gives, as the initially observed adsorbate, [Os₃H(CO)₁₀(µ-O–oxide)](2)(on silica) or [Os₃(CO)₁₀(µ-O–oxide)(OH)](3). Following analyses of the model compounds [Os₃XY(CO)₁₀](X = Y = H; X = H, Y = OMe; X = Y = OEt), the structure of this species on alumina was investigated by Os L(III) edge extended X-ray absorption fine structure. Both (2) and (3) can be fitted to the data, but the latter affords more reasonable Debye–Waller factors. Subsequent pyrolysis generates a mixture of [Os(CO)₃]_n and [Os(CO)₂]_n species. This process is reversible on silica. Attempts to simulate the v(CO) absorptions of a ¹³CO-enriched version of the pyrolysis products to simple mono- and di- nuclear models were unsuccessful. This evidence, together with the electronic absorption spectra of these materials, suggests a trimeric array as the smallest likely unit, probably with some metal–metal bonding. A by-product from the reaction with alumina and titania was [Os₃H(Cl)(CO)₁₀]. Interaction of [Os₃(CO)₁₂] with a hydroxylated cleaved mica crystal in refluxing n-octane gave rise to the pyrolysis products [Os(CO)₃]_n and [Os(CO)₂]_n.