H-bonding interactions of the cytosine-modeling compounds 4-aminopyridine and 4-aminopyrimidine with water are investigated with the use of a combined experimental matrix-isolation FT-IR and theoretical abinitio method. Both bases appear exclusively as the amino tautomers in low-temperature Ar matrices, in accordance with large ab-initio predicted energy differences with the rare imino forms. The most stable complex of water with 4-aminopyridine is the H-bonded N-1 ... HO-H structure, but the energy difference with the alternative N-H ... OH2 complex is calculated to be only 6.7 kJ/mol. Both complexes are observed in the Ar matrix. In the case of 4-aminopyrimidine, the closed N-H ... O-H ... N-3 structure containing two H-bonds is found to be significantly more stable than the singly H-bonded structures N-1 ... HO-H and N-H ... OH2 by 8.0 and 10.2 kJ/mol, respectively. The previously suggested relation between the scaling factor for the theoretically-calculated v(OH)(b) mode of the H-bonded water and the proton affinity value of the proton-acceptor site in the base is developed further.