The UV-absorption, fluorescence excitation and emission spectra of the alkaloids eudistomin N (6-bromo-nor-harmane) and eudistomin O (8-bromo-nor-harmane) were described. In order to perform a comparative analysis, we also studied other bromo-β-carbolines and the corresponding non-substituted-carboline. Thus, 6-bromo-, 8-bromo-, 6,8-dibromo-, 3,6-dibromo- and 3,6,8-tribromo-derivatives of nor-harmane, harmane and harmine were studied. These studies were performed in EtOH and in EtOH+1% perchloric acid solutions (pa). Furthermore, fluorescence quantum yields (ϕ f ) in acetonitrile and acetonitrile+1% perchloric acid solutions at 298K were measured. The HOMO and LUMO energy, the positions (λ max ) and oscillator strength (f) of the 1 S 1 ← 1 S 0 band for all the neutral and protonated β-carbolines studied was calculated and compared with the experimental data. The pK a values in aqueous solution for eudistomin N and O (6-bromo- and 8-bromo-nor-harmane), for 6-bromo-, 8-bromo- and 6,8-dibromo-harmane, and for 6-bromo- and 8-bromo-harmine were spectrophotometrically measured (pKa(H2O)). The change of the acid–base character of these compounds on going from the ground state (pK a ) to the first electronic excited singlet state (pKa∗) as ΔpKa=pKa∗-pKa=0.625Δν¯/T, in ethanol solution at 298K were calculated (ΔpK a(EtOH) ). Proton affinities (PA) for all the compounds studied defined as minus the enthalpy change of the reaction M+H + →MH + (gas state) were calculated. Basicity relative to pyridine (ΔH rPy ) defined as the enthalpy change of the isodesmic reaction MH + +Py→M+PyH + (gas state) was also calculated. The effect of bromine as substituent on the properties of the β-carboline moiety in nor-harmane, harmane and harmine is discussed.