The mechanism for the substitution reactions of halopentaaquachromium(III) complexes in the series, [CrX(H2O)5]2+, where X=F−, Cl−, Br−, or I− have been investigated using density functional theory. Several different mechanistic pathways were explored including associative interchange (Ia), dissociative (D) and the associatively activated dissociation mechanisms (Da). The lowest overall activation enthalpy (ΔH‡) obtained for the fluoride system is for the Ia pathway, with immediate proton abstraction leading to the formation of HF and the conjugate base. For the chloride, bromide and iodide systems the Da pathway has the lowest ΔH‡ values. Activation enthalpies determined at the PBE0/cc-pVDZ level, in aqueous solution (PCM), are in excellent agreement with the experimental results (MAD is 1.0kJmol−1). Reaction profiles were analyzed in terms of activation volumes to explain the observed trends.