Bacterial alcohol oxidation frequently proceeds via respiratory-chain linked dehydrogenases which have pyrrolo-quinoline quinone (PQQ) as their coenzyme. These so-called quinoprotein alcohol dehydrogenases have originally been isolated from methylotrophic bacteria and in these cases they are known as methanol dehydrogenases (EC 1.1.99.8). In this report we describe the results of mechanistic studies using monomeric as well as dimeric enzyme forms. Recently Mincey et al. (1981) put forward a view on the mechanism of action which is quite different from the one reported by us (Duine and Frank, 1981). Their model is based on an enzyme form containing the semiquinone of PQQ, reacting with substrate and producing a three-electron reduced form of PQQ. In our view, only the fully oxidized enzyme form reacts with substrate. In order to settle this controversy, inactivation experiments were performed with cyclopropanot, using dimeric as well as monomeric methanol dehydrogenase (the latter enzyme form was used in the experiments by Mincey et al., 1981). From the results the following conclusions can be drawn: cyclopropanoI acts as a suicide substrate for both enzyme forms. only enzyme molecules in the fully oxidized form react with cyclopropanol. titration experiments reveal that there is no proton production and no electron acceptor consumption during the inactivation step. in fully inactivated enzyme, all PQQ is converted into an adduct with suicide substrate. in dimeric enzyme, the two catalytic sites act independently (50% inactivation is achieved with stoichiometric amounts ofcyclopropanol and enzyme). From these results and from evidence obtained from ESR experiments (De Beer et ak, I983) it is clear that reaction of substrate or suicide substrate only occurs with fully oxidized enzyme molecules. In this respect, no difference is observed between monomeric and dimeric enzyme preparations. All conclusions are in agreement with and substantiate our reported model for the catalytic cycle of the enzyme (Duine and Frank, 1981).