The hydrogen bond formation between 6-methyl purine and cytosine has been studied at room temperature. The absorption and fluorescence emission spectra of 2.10−5μ 6-methyl purine solutions containing different amounts of cytosine in the range of 2.10−4−3, 5.10−4μ were investigated. The association constant for hyrogen bonded complex formation between 6-methyl purine and cytosine was found to be 1, 1.103μ−1from absorption difference spectra. The same association constant in the excited state was found to be 3, 9.103μ−1from fluorescence spectra.