The solubility of alloxazine in aqueous solution at pH 4 (10–4 molar HCl) and pH 10 (10–4 molar NaOH) at room temperature is determined by absorption spectroscopic characterization. Knowledge of the solubility is needed for sample preparation and quantitative spectroscopic solution characterization. Samples of different in-weight concentration were prepared and absorption spectra were measured versus storage time. The solubility limit concentration Csol was determined by the crossing point of the linearized absorption coefficient dependences on in-weight preparation concentration below and above the solubility limit. Values of Csol= 9.05 ± 1 μM and 14.5 ± 1 μM were determined for alloxazine in aqueous solution at pH 4 and pH 10, respectively.