The thermolysis of 2-oxetanones (/3-lactones) leading to carbondioxide and olefins has been studied for the first time from the theoretical point of view by means of the semiempirical SCF-MO methods AMI, MNDO, and MINDO/3 at the RHF level. The reaction of the parent 2-oxetanone is predicted by all three methods to be concerted but highly asynchronous, taking place through a transition state with high zwitterionic character where all ring atoms liein the same plane. An AM1-HE-CI study of the same process shows the absence of diradical character along the reaction path. The AMI calculated enthalpy of activation and enthalpy of reaction are the closest to the experimental ones. The process has also been examined on a set of 25 diversely substituted 2-oxetanones by the AMI method, the experimentally observed substituent effects being well reproduced by the calculations. The decarboxylation of 2-oxetanone protonated at the carbonyl oxygen atom has been studied as a model for the acid-catalyzed thermolysis of 2-oxetanones, finding that in this case the reaction takes place stepwise, through a carbocationic intermediate whose preferred conformation allows an interpretation of the observed stereochemical outcome of the reaction under acid catalysis. A reaction analysis by correlationof localized molecular orbitals has been performed on both the purely thermal and the acid-catalyzed processes, allowing the visualization of the electronic changes that takeplace along the reaction coordinate. Some simple ways of using the bond index concept for the studyof chemical reactions are proposed. Application of these ideas to the thermolysis of 2-oxetanones reveals the existence of significant correlations between the parameters derived from bond indexanalysis and calculated enthalpies of activation.