Gilbert's research work is in two main areas, in each of which he is internationally recognised as a leading expert. The first is kinetics of the industrially important emulsion polymerisation processes, in which significant advances in development and applications of theoretical methods have been made. The second is kinetics of gas-phase unimolecular reactions, in which the goal of development of reliable ab initio theoretical models for these processes that can be used predictively has been considerably advanced. In both areas, extensive experimental work has been carried out in collaboration with physical chemists to test predictions of theory.