An effective low cost method for the photochemical production of vitamin D-2 has been studied, including a 200 L batch photoreactor and a continuous reactor. Following optimization of the irradiation time (550 min), initial concentration of ergosterol (1.4 mg/mL) and the number of lamps employed (16) for the batch reactor, the conversion of ergosterol is ca. 50 %, and the selectivity yield of previtamin D2 is ca. 84 %. The corresponding data for the continuous reactor are 73 % and 62 %, respectively.