Using published data, the symmetry effect on the entropy of fusion, Delta S-f, of rigid organic molecules has been investigated by comparing structural isomers. Because Delta S-f is dependent on the enthalpy of fusion, Delta H-f, [A.S. Gilbert, Thermochim. Acta 339 (1999) 131-142] this dependence must be allowed for as Delta H-f values can vary quite widely. When this is done, it is found that more symmetric isomers usually have lower than expected values of Delta S-f, in relation to Delta H-f, than their less symmetric counterparts. This is the reason that more symmetric isomers generally melt at higher temperatures, a fact noted by many workers over the years. (C) 2006 Elsevier B.V. All rights reserved.