Firefly bioluminescence displays a sensitivity to pH changes through an alteration of the energy of the emitted photon leading to yellow-green light above pH 6.5 and red light below this value. Calculations using the fragment molecular orbital method have been performed on the active site of the luciferase enzyme from the Japanese firefly Luciola cruciata in order to investigate both the importance of different protonation states and tautomeric forms of the lumophore, oxyluciferin, and the role played by protonation of the active site AMP molecule. The results suggest that whilst an equilibrium between several protonation/tautomeric states of oxyluciferin is possible, a single oxyluciferin species (the phenolate–keto form) may be mostly responsible for both emission colours, with changes in polarization by the active site caused by protonation of the AMP molecule playing an important role in mediating the pH-dependent shift.