There is an increasing interest in the study of NO chemical affinities of organic nitrites, for the biological and physiological effects of organic nitrites seem to be due to their ability to release NO. In this paper, NO chemical affinities of ten substituted benzyl nitrites were determined by titration calorimetry combined with a thermodynamic cycle in acetonitrile solution. The results show that ΔH _het(O-NO)s of benzyl nitrites are substantially larger than the corresponding ΔH _homo(O-NO)s, suggesting that these O-nitroso compounds much more easily release NO radicals by the O-NO bond homolytic cleavage. It is believed that the structural and energetic information disclosed in this work should be useful in understanding chemical and biological functions of organic nitrites.