The electronic and structural properties of 2-hydroxypyridine (2HP) and 2HP (.) (NH3)(n) (n = 1-4) and their tautomers in the excited state are studied using the CIS and TD-B3LYP calculations. The role of ammonia in acting as a bridge between tautomers was also discussed. Specific attention is paid to resolve the threshold size for excited state proton transfer in these clusters. It was found that at least three ammonias are needed to produce a stable excited ion-pair clusters. The ammonia catalyzed enol-keto tautomerization reactions involve concerted proton transfer mechanism. These behaviors are found also in the ground state in our previous study, with DFT calculations performed. (c) 2006 Elsevier B.V. All rights reserved.