The asymmetric unit of the title compound, C(12)H(9)N(2) (+)·ClO(4) (-)·C(6)H(6)N(4)O·2H(2)O, contains a monoprotonated 4,7-phenanthrolinium (47phen) cation, a perchlorate anion balancing its charge, a neutral mol-ecule of 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO) and two inter-stitial water mol-ecules. In the crystal structure, the acidic H atoms of 47phenH(+) and HmtpO form strong hydrogen bonds with the water mol-ecules, which in turn act as hydrogen-bond donors, forming links between them and towards the carbonyl O atom of HmtpO, the non-protonated N atom of 47phen(+) and one of the O atoms of the anion.