Copper sulfate pentahydrate dehydration is studied by using variable temperature diffuse reflection infrared Fourier transform spectroscopy (VT-DRIFTS). In a static environment, dehydration proceeds sequentially from the pentahydrate to the trihydrate and then to the monohydrate. When helium purge is employed while heating samples, the monohydrate is formed directly from the pentahydrate. Infrared spectral features show that in a static environment, lattice water molecules are involved in coordinated motions, which are absent when purge is employed. Differences between crystal structures obtained in the static and purge environments are explained by equilibrium processes that depend on local water vapor concentration and lattice water environments. (C) 2011 Elsevier B.V. All rights reserved.