For model reduction of chemically reacting systems with fast reversible reactions, reaction invariant compositions, as introduced by Doherty and co-workers for reactive distillation processes, can be used. Reaction invariant compositions are obtained from a nonlinear transformation of the original composition variables. The transformation requires the choice of a suitable set of reference components. In the present paper it is proven, that such a set of suitable reference components always exists. Furthermore, a systematic procedure for computing such a feasible set is given. (c) 2007 Elsevier Ltd. All rights reserved.