Kinetics studies on the interaction between lead dioxide and methylcobalamin indicate a pattern of two parallel first‐order reactions with rate constants of 1.02 x 10‐3s‐1and 5.33 x 10‐5s‐1. The relative importance of the two reaction pathways seems to depend on the available surface of lead dioxide. Acetone, methane and methanol form as the principal gaseous products. Traces of tetramethyllead form, and substantial quantities of lead dissolve in the aqueous layer. The product distribution is consistent with a mechanism involving formation and decomposition of methyllead triacetate. Other insoluble metal oxides also follow this pattern in their reaction with methylcobalamin.