Par G. PIRLOT (Liege) et E.?V. ROUIR (Gembloux) S D M M~I .-The tlc4erniination of bitamin D2 in the presence of tliflrrcnt irradiation prndiicts of ergostcrol is rather difficult b y simple iiltrit-uiolct nbsorption spctroacopy, dur to the suprrposition of t h e ,ihqorption I)anrI* of Ihrse substances.It is thus necessary to resorl lo cheiriiral transforninlioiis i i i order In get producls wich absorb in different wavclcnght rangrh for the ~t~v r r a l substances in presencc. The proccdurr we have worked out; basrd on a reaction given by Sobel and ctworkcrs, i4 siiitrd to the determination of vitamin D, prepared from rrgo4rrol.