Abstract—–The influence of temperature, pH, salts, and reactant concentrations on the biolumin‐escent oxidation ofCypridinaluciferin catalyzed byCypridinaluciferase indicates a highest quantum yield φ (einsteins per mole of luciferin oxidized) of 0.31 in H2O, or 0.33 in 99% D2O. With the aid of data on fluorescence of the light‐emitting oxyluciferin‐luciferase complex, and of oxyluciferin in diglyme, partial explanations are suggested for the observed variations in φ, including the relatively low φ, of 0.03 for chemiluminescence of luciferin in organic solvents, wherein a different pathway of luciferin degradation, as indicated by chromatographic evidence, results in much less population of the e