It is well known that the carboxylic acids form dimeric species in dilute solution likewise as 2-pyrrolidinone, a cis lactam, with as a consequence for the latter that two bands due to CO absorption are observed in solid state, the first at 1680 cm−1in infrared, the second at 1653 cm−1in Raman (1, 2). Checking the CO frequencies of the imidazolidinetrione, we observed also the no-coincidence of the frequencies in infrared and in Raman (3). This fact which was also pointed out by ALEMAGNA and LORENZELLI (4) was attributed to experimental errors. It is interesting to examine which is in CO region the spectral behaviour of imides (structure I) which are forming dimeric species in solid state (structure II) (5, 6).