Two excited state proton transfer mechanisms of 3-hydroxyisoquinoline (3HIQ) in cyclohexane and acetic acid has been investigated by time-dependent density functional theory (TDDFT). Spectral and photo physical properties of 3-HIQ in various protic/aprotic solvents were studied. Phototautomerization of 3-HIQ has been reported. Oxo-hydroxy tautomerism and phototautomerism of 3-HIQ has been studied using the matrix-isolation technique.