The crosslinking reactions of ethylene vinyltrimethoxy silane (EVS) copolymers at processing temperatures have been studied. Samples were heated in inert atmosphere with or without water at 130–350°C. The crosslinking reactions were followed by determination of the gel content and by analyzing the structural changes using FT-IR. As expected, no gel could be observed in pure EVS after treatment in nitrogen, whereas 30% was obtained if the nitrogen was presaturated with water. Addition of a catalyst, dibutyltindilaurate, increased the reaction rate considerably and a final gel content of 70–75% was obtained above 300°C. In contrast, neither water nor catalyst was needed for the formation of gel in a terpolymer of EVS containing butylacrylate as well. During heat treatment the acrylate units formed carboxylic acid, and blends of EVS and an ethylene acrylic acid (EAA) copolymer were therefore used as a model to study the reactions in detail. Despite absence of water these blends started to form gel at low temperatures, 150–200°C. This could mainly be related to internal production of water due to reaction between carboxylic groups leading to anhydride. Both cyclic and noncyclic anhydrides were detected by FT-IR analysis. A larger amount of EAA in the EVS–EAA blend increased the rate of gel formation and decreased the onset temperature, but no corresponding change in the formation of noncyclic anhydride was observed. Another kind of crosslink, formed by reaction between carboxylic acid and silanol groups, can possibly be one explanation to this behavior.