The kinetics of the isomerization of butenes over the Bi2O3-MoO3 (koechlinite) catalyst were investigated in recirculation and in pulse expts. At 150-250 Deg, isomerization prevails over the oxidn. to butadiene. In the recirculation expts., isomerization proved strongly inhibited by butadiene, and the apparent activation energies for isomerization were high (17-19 kcal mole-1). In the pulse expts. where diene inhibition is nonoperative, low activation energies (2-4 kcal mole-1) for isomerization were obsd. Isomerization and oxidn. occur on similar combination of the B and A centers as postulated by Matsuura. This was confirmed by the action of NH3 in a temp. range where isomerization and oxidn. are of comparable rate. Both reactions proved to be strongly inhibited.