An annular flow photochemical reactor illuminated by UV and green (524 nm) light emitting diodes (LEDs) was characterised by a chemical actinometer. Very high efficiency of absorption of photons, most likely promoted by the specific orientation of LED elements in the reactor, was calculated based on the measured actinometry results. Generation of singlet oxygen mediated by nanoporous silicon under illumination by Ar+ laser, UV and green LEDs was demonstrated by indirect measurement of suppression of porous Si photoluminescence, and by direct measurements of singlet oxygen luminescence. The efficiency of reactor in singlet oxygen mediated reactions was tested using reaction of decomposition of diphenylbenzofuran. Estimated quantum yield of chemical reaction is about 34%. (C) 2007 Elsevier B.V. All rights reserved.