Density functional theory calculations were applied to study a set of isomerization and decomposition reactions that zinc dialkyldithiophosphate (ZDDP) engine oil antiwear additives may take part in. The products of these reactions comprise a set of chemical species that can lead to the formation of poly(thio)phosphate films that are thought to be responsible for the antiwear protection offered by ZDDPs. The specific reactions examined involved either intramolecular alkyl group transfer within ZDDP or the intramolecular elimination of olefins from this molecule. A series of substituents were employed to examine how the nature of the substituent on the ZDDP molecule affects the thermodynamic and mechanistic details of these reactions, which in turns may have ramifications regarding various aspects of antiwear film formation. It was found that qualitative and quantitative aspects of these reactions were markedly different when hydrogen atoms were used as substituents instead of alkyl groups. The details of the reactions for the H-substituted system indicated that the precursors to the antiwear films should not be formed. When alkyl groups were employed as substituents, the energetic details of the reactions considered in this study exhibited a dependence upon the nature of these groups. An examination of these details revealed that straight-chained primary alkyl ZDDPs should decompose through reactions involving alkyl group transfer, while secondary and branched primary alkyl ZDDPs should primarily undergo olefin elimination reactions to form precursors to ZDDP antimear films. The details of the reaction pathways that these systems follow to form the precursors shed light on the observed byproducts of antiwear film formation.