Tautomeric equilibria of amino forms of 2,6-diamino-8-azapurine and 8-aza-iso-Guanine is revealed by DFT computations. The most populated tautomer of 2,6-diamino-8-azapurine, in agreement with available experimental data, is protonated at position N(9). The lowest free energy tautomer of 8-azaiso-Guanine is protonated at positions N(3) and N(8). For biologically more important tautomer N(9)-H, probability of N(3) protonation is higher than N(1). This result, observed also for iso-Guanine, shows reversed probability of protonation at positions N(3) and N(1), compared to Guanine and decreased specificity of pairing of 8-aza-iso-Guanine with iso-Cytosine should be expected. Computed electronic excitation energies well match available experimental data. (C) 2015 Elsevier B.V. All rights reserved.