The cocyclization of ethyne and nitriles to 2-substituted pyridines by means of cobalt(I) complexes as catalysts is accelerated by light. Different types of pyridines were synthesized in this way. Maximum light absorption by the catalyst was found to lie between 400 and 550 nm, i.e. within an optimal range of the solar spectrum. The finding that water is usable as a medium for the reaction opens the way for experiments using a commercial solar plant. The experimental work is described. The advantage of the proposed solar chemical system may be a favorable energy balance.