Abstract. Given a signature of basic operations for a computational effect such as side-effects, interactive input/output, or exceptions, we give a unified construction that determines equations that should hold between derived operations of the same arity. We then show how to construct a canonical model for the signature, together with the firstorder fragment of the computational λ-calculus, subject to the equations, done at the level of generality of an arbitrary computational effect. We prove a universality theorem that characterises the canonical model, and we recall, from a previous paper, how to extend such models to the full computational λ-calculus. Our leading example is that of side-effects, with occasional reference to interactive input/output, exceptions, and nondeterminism.