Let Q be a smooth nowhere-zero w-form on a non-compact ndimensional manifold Y. We study the homology of the group Diff n Y, consisting of all diffeomorphisms of Y preserving Q, provided with the discrete topology. In particular, if Q is the standard volume form dx { A ... A dx n on R", and if n # 2,3, we show that the natural maps #Diff n R" c > £Emb n R" -^-> BTJ induce isomorphisms of integral homology. Here Emb n R" is the discrete monoid of smooth embeddings of R" preserving 0., and J5F n " is the Haefliger classifying space for codimension-n smooth foliations with a smooth non-vanishing transverse closed w-form. (Analogous statements for the group of all diffeomorphisms of R" were proved by Segal in [16] with no restriction on dimension.) It follows that if n # 2, 3 the natural map j5Diff n R" -> l?SO(w), where SO(w) is given its usual topology, induces an isomorphism on integral homology #; for 0 ^ i < n, and a surjection on H n with kernel isomorphic to R. If Y is a non-compact manifold which is diffeomorphic to the interior of a compact n-dimensional manifold with boundary, and if every end of Y has infinite Q-volume, we show that providing that n # 2,3. Here Diff fi0 <= Diff n denotes the subgroup consisting of elements smoothly isotopic to the identity.