The combined experimental and theoretical ab initio method for investigation of hydrogen-bonded complexes between water and molecules modeling cytosines has been applied to 1-CH3-cytosine. This compound occurs in Ar predominantly in the amino-oxo form, but traces of the imino-oxo E tautomer are also present in accordance with the ab initio prediction. An approximate value of 7.2 is obtained for the tautomerization constant K-T(a-o/i-o) using the infrared intensity measurement. The most stable H-bonded complex of the abundant a-o tautomer with water is the closed N-3...H-O...H-N-14 structure, which shows cooperativity between the two H-bonds. Less stable complexes are the closed N-3...H-O-H...O-7 complex and the open N-14-H...OH2 and N-14...H-OH structures, In the experimental FT-IR spectra, the closed N-3...H-O...H-N-14 and open O-7...H-OH complexes are identified based on characteristic absorptions predicted by ab initio calculations or based on earlier obtained experimental results for similar structures, Despite some trace amounts of the i-o E tautomer in the Ar matrix, H-bonding of this rare form with water was detected in the form of the closed N-14...H-O...H-N-3 structure, A preliminary correlation between the frequency shifts of the bonded water stretching mode and the proton affinities of the C=O group is presented for the open C=O...H-OH complexes of 1-CH3-cytosine and some related compounds.