Abstract: Various embodiments of photoreactors are disclosed which have at least two irradiation chambers with a window therebetween. Ultraviolet radiation is introduced into one of the chambers at a side opposite the window so that it passes through that chamber, through the window and into the other chamber. The fluid medium to be purified is passed through the chambers and subjected to the radiation while in the chambers. The flow of the medium is through the chambers in series. The chambers are optimized in depth with respect to efficiency in terms of the flow-dose rate. In a two chamber photoreactor traversed by parallel radiation the maximum is at a total absorption of about 70 percent and at an absorption of about 27 percent in the chamber immediately adjacent to the radiation source. In an annular two chamber photoreactor with an elongate radiation source extending along the axis thereof the corresponding data are 45 to 65 percent and 16 to 25 percent, respectively.