Ab initio and density functional theory (DFT) calculations predict that intramolecular homolytic substitution by alkyl radicals at the selenium atom in seleninates proceeds through smooth transition states in which the attacking and leaving radicals adopt a near collinear arrangement. When forming a five-membered ring and the leaving radical is methyl. G3(MP2)-RAD calculations predict that this reaction proceeds with an activation energy (Delta E(1)(t)) of 30.4 kJ mol(-1). ROBHandHLYP/6-311++G(d,p) calculations suggest that the formation of five-membered rings through similar intramolecular homolytic substitution by aryl radicals, with expulsion of phenyl radicals, proceeds with the involvement of a hyper-valent intermediate. This intermediate further dissociates to the observed products, with overall energy barriers of about 40 kJ mol(-1). Homolytic addition to the phenyl group was found not to be competitive with substitution, with a calculated barrier of 57.6 kJ mol(-1). This computational study provides insight into homolytic substitution chemistry involving seleninates.