Thermal and photochemical stability (Φ_R), room temperature UV-vis absorption and fluorescence spectra, fluorescence quantum yields (Φ_F) and lifetimes (τ_F), quantum yields of hydrogen peroxide (Φ_H2O2) and singlet oxygen (Φ_Δ) production, and triplet lifetimes (τ_T) have been obtained for the neutral and protonated forms of 6-chloroharmine, 8-chloroharmine and 6,8-dichloroharmine, in aqueous media. When it was possible, the effect of pH and oxygen concentration was evaluated. The nature of electronic transitions of protonated and neutral species of the three investigated chloroharmines was established using Time-Dependent Density Functional Theory (TD-DFT) calculations. The impact of all the foregoing observations on the biological role of the studied compounds is discussed.