I of p-(1R,3S)3-thioanisoyl-1,2,2-trimethylcyclopentanecarboxylicacid has been determined and is compared with that of polymorph II that was previously described. Polymorph I is very different at the level of the carboxyl group. It does not present disorder and the values found for the C-O bonds correspond exactly to single and double bond lengths. In addition, the carboxyl groups of the two molecules