In this paper we generalize the Fenichel theory for attracting critical/slow manifolds to fast-reaction systems in infinite dimensions. In particular, we generalize the theory of invariant manifolds for fastslow partial differential equations in standard form to the case of fast reaction terms. We show that the solution of the fast-reaction system can be approximated by the corresponding slow flow of the limit system. Introducing an additional parameter that stems from a splitting in the slow variable space, we construct a family of slow manifolds and we prove that the slow manifolds are close to the critical manifold. Moreover, the semi-flow on the slow manifold converges to the semi-flow on the critical manifold. Finally, we apply these results to an example and show that the underlying assumptions can be verified in a straightforward way.• We show that the fast-reaction systems are well-posed and prove the convergence of solutions as the parameter ε → 0 (see Section 3).• We generalize Fenichel's theorem for fast-reaction systems in infinite-dimensional Banach spaces.Under suitable assumptions we prove the existence of a C 1 -regular slow manifold that is close to the critical manifold in a suitable sense. Moreover, we show that the critical manifold attracts solutions and that the flows on the slow manifold converge to the flow on the critical manifold. To conclude this part we apply the theory to an example (see Section 4).• Lastly we present a brief overview on the theory of C 0 semigroups and their application to abstract Cauchy problems (see Appendix A).