AbstractIrradiation of ergosterol, lumisterol, tachysterol or previtamin D at room temperature for a long time brings about a considerable lowering of the extinction in the region 220‐300 mμ, whereas this phenomenon is not observed on irradiation of these compounds at 80°K in solid solution. From this it may be deduced that in the course of the photochemical reaction at room temperature ‐ besides the well‐known isomerisations – di‐ or poly‐merisations take place. A solution of tachysterol in liquid propane also shows the lowering of the extinction on irradiation. This indicates that the high viscosity of the medium rather than the low temperature is responsible for the non‐occurrence of lowering of the extinction in solid solution at 80°K.It is found that the photochemicalcis/transisomerisation of previtamin D into tachysterol is not ‐ or only partially ‐ inhibited by lowering the temperature to 80°K either in solid or in liquid solution. The conversion of tachysterol into previtamin D, on the other hand, did not occur on irradiation in liquid propane solution at 80° K. This is in agreement with theoretical considerations which were recently worked out in this laboratory for the mechanism of thecis/transisomerisation in the case of stilbene. These studies lead to the assumption that for the conversion of the photo‐excitedtrans‐isomer (in the singlet state) thermal energy is required to provide the (rather small) energy of activation. The excitedcis‐form may isomerise into thetrans‐form without