Ab initio calculations at the B3LYP/6-31G(d) level and scaled particle theory, combined with entropies of activation derived from experimental Arrhenius A factors, were applied to examine the origin of the loss of gaseous-phase entropy of a substance upon solution. Eight reactions in water were analyzed: H atom reacting with methanol, ethanol, 2-propanol, methanediol and ethylene glycol; and methanethiol reacting with the radicals, methyl, hydroxymethyl, and 2-hydroxy-2-propyl. The results suggest that the observed entropy loss is entirely due to changes in the solvent. The dominant factor is a loss of entropy due to cavity formation. This is partially offset by a corresponding increase in the disorder of the H-bonding network in the case of the larger species.