These rules describe a system for the representation of models of reaction mechanisms in a linear form, using standard computer characters. A model of the reaction mechanism is described in a sufficiently detailed manner that most of the important features can be represented, and the notation can be related to the conventional diagrammatic representation of reaction mechanisms. The detail given is however such that it is not intended that the representation should be used in speech. The model is described primarily in terms of bond making and bond breaking steps , but the representation allows the encoding of much other information. It allows the representation of single reaction steps or reaction sequences, and should specify exactly how the reagents are converted into the products. It is primarily designed for thermal organic reactions, but is not in principle limited to that field.