Simultaneous intersystem crossing (ISC) and intramolecular hydrogen transfer (HT) reactions were studied experimentally in three triplet carbenes: 1-(3-thienyl)-ethylidene (C1), 1-phenylethylidene (C2) and 1-(2-naphthyl)ethylidene (C3). The reaction was observed in C1 at 10 K, in C2 when heated to 65 K, and not at all in C3 despite heating, with compelling evidence of tunnelling both reactions. Here, we have used instanton theory to study various possible reaction mechanisms, including a sequential process in which HT is followed by ISC or a concerted mechanism in which they occur simultaneously. In this talk, I describe instanton theory which is a versatile method to describe tunnelling in both adiabatic (HT) and nonadiabatic (ISC) reactions. In all carbenes, we find that the concerted mechanism is responsible for the reaction at low temperatures, while the sequential mechanism dominates at higher temperatures. In effect, there exists a crossover temperature where the reaction mechanism switches. This work describes a reaction mechanism which involves switching electronic states while tunnelling underneath a barrier, and shows that temperature can be used to tune the reactivity in triplet carbenes. I will end with an overview of the reactions that have been studied using instanton theory.