The low-lying triplet states of 10-hydroxybenzo[h]quinoline (HBQ) and its halogenated derivaties 7,9-diiodo-10-hydroxybenzo[h]- quinoline (DIHBQ) in an excited-state intramolecular proton-transfer (ESIPT) process have been investigated. For DIHBQ, which is enhanced by the intramolecular heavy atom effect, the proton-transfer tautomer (i.e., the keto form) phosphorescence that is maximized at 735 nm (tau(p) = 1.75 mus) was resolved in a 77 K methylcyclohexane (MCH) glass. Further transient absorption and 10, sensitization experiments allow us to deduce the population yield and radiative decay rate of the keto triplet state to be 0.85 and 8.8 s(-1), respectively. Upon increasing the excitation intensity, photolysis reactions were observed for both HBQ and DIHBQ in the MCH glass. The reversibility of the photolysis reaction throughout a thawing and freezing cycle led us to conclude that the rupture of an intramolecular hydrogen bond through the excessive energy dissipated does occur. The product exhibits a non-hydrogen-bonding type of enol emission that would otherwise be inaccessible in the hydrogen-bonded enol form because of the ultrafast ESIPT. Accordingly, relative energy levels in different spin manifolds are established during a proton-transfer cycle. The keto-enol reverse proton transfer in the lowest triplet manifold was estimated to be endergonic by similar to7.42 kcal/mol.