The association reactions of methylated acetone with acetone and dimethyl ether were studied over an extended temperature range (30-250 "C) by high-pressure mass spectrometry. In addition, the association reactions of various deuterium-substituted analogues of both the reactant ion and neutral molecules were investigated. Two distinct isomeric forms are apparent from the examination of the behaviour of the association equilibrium constant as a function of reaction temperature: (i) a low-temperature, covalently bound isomer characterized by larger -AH" and -AS" values and significant equilibrium isotope effects on the entropy of association; and (ii) a high-temperature, electrostatically bound isomer of lower -AH" and -AS" values and with insignificant equilibrium isotope effects. The equilibrium isotope effects and larger -ASo values are more pronounced for the methylated acetone-dimethyl ether adduct than for the less sterically congested methylated acetone-acetone adduct. Molecular models of the low-temperature isomers show severe restriction or complete loss of internal rotations and is the origin of the observed equilibrium isotope effects. No such phenomenon is expected or observed in the more loosely bound high-temperature, electrostatic isomers. Potential energy surfaces for the transfer of methyl cation from methylated acetone to a second acetone molecule and to dimethyl ether are proposed.