The adsorption of 1-butene and butadiene on Bi2Mo2O9 and on silica-supported Bi9PMo12O52 catalyst was measured. Both catalysts (in which the Erman phase is the major component) show weak single-site adsorptions of 1-butene and butadiene and a slow but strong adsorption of butadiene. Thermal decomposition of the Erman phase into Bi2O3 · 3MoO3 and Bi2O3 · MoO3 runs parallel with a change of the weak single-site type of adsorption into a dual-site type of adsorption, well known as belonging to the koechlinite phase. The silica-supported catalyst is thermally more stable than the Bi2Mo2O9 compound. The supported catalyst is found to be partially unstable if exposed to reduction-reoxidation conditions at 470 °C. Suggestions are given as to the role of P in silica-supported Bi-Mo catalyst.