The basic "hydrolysis" of ethylene-vinyl acetate copolymers (EVA) leads to ethylene-vinyl alcohol copolymers (EVOH) or ethylene-vinyl acetate-vinyl alcohol terpolymers which can be used for their surface properties or for further chemical modification. The controlled basic solvolysis of EVA takes place via a complicated nonstoichiometric mechanism. The solvolysis by anhydrous methanol catalyzed by sodium methoxide has been investigated thoroughly. This reaction is capable of fully converting acetate groups; the solvolysis of an acetate group next to another acetate (in EVV triads) is faster than that for isolated vinyl acetate (EVE triads) and proceeds immediately to the neighboring acetate group. The rate constants of each elementary step have been determined with the help of numerical simulations of kinetics data. In particular, the forward rate constant, k(+), of the S(N)2 attack of the methoxide ion on vinyl acetate strongly depends on solvent quality (methanol content), i.e., on polymer coil expansion. When sodium hydroxide is used as a base, the reaction system is more complex because of the additional steps and equilibria involving water. The most prominent difference with respect to anhydrous conditions is partial solvolysis of acetate groups, even when fully converted. Indeed, the reaction stops when the reactive species have been fully converted into acetate ions of low reactivity. The numerical simulation of the kinetics agreed with the experimental data, but the large number of reaction steps and equilibria did not allow reliable determination of the rate constants.