In case of Thiocarlidum (1) the thermoanalytical examination showed an endothermic transformation without loss of weight. This occured in samples recrystallized from cold polar solvents and the transformation proved to be non-reversible. The UV and IR spectra of the two samples differed characteristically and X-ray studies showed that the two modifications (in the following aand ßmodifications where the latter is produced as a result of thermal transformation) have different crystal structures. This can be considered as a ground for the differences in the tabletting characteristics of the two forms 1. According to the UV and IR spectra it is to be seen that polymorphy alone does not suffice to interpret the experimental evidences. Since the two modifications have the same spectra in all the solvents examined, and the thermal transformation is not accompanied by a change of weight, structural isomerisation, tautomerisation and conformational change, respectively, could be considered as a ground of the anomalies found.