We present a simple formulation of Assumption-Commitment reasoning using CSP. In our formulation, an assumption-commitment style property of a process SYS takes the form COM [= SYS || ASS, for some 'assumption' and 'commitment' processes ASS and COM. We state some proof rules that allow us to derive assumption-commitment style properties of a composite system from corresponding properties of its components, given appropriate side conditions. Most of the rules have a superficially appealing `homomorphic' quality, in the sense that the overall assumption and commitment processes are composed similarly to the overall system. We also present a `non-homomorphic' rule that corresponds quite well to proof rules of established assumption-commitment theory. The antecedants and side conditions are expressed as refinements that can be checked separately by the refinement-style model checker FDR. Examples are given to illustrate application of our theory.