A monolayer model extended to two energy levels (monolayer model 2) has been developed to fit the adsorption isotherms of acid blue 9 on Spirulina platensis (Sp LEB-52). These isotherms were obtained at different values of pH and different temperatures. The adsorption isotherm modeling was based on a statistical physics approach. The evolutions of stereographic and energetic parameters involved in the model equation were presented in this work. The obtained values of nd(1) and nd(2) are always higher than unity, indicating that the adsorption of acid blue 9 molecules on Sp LEB-52 is a multimolecular adsorption. The highest adsorption capacity (Q(s) = 1251.8 mg g(-1)) is signaled at pH = 4 and T = 298.15 K. Furthermore, we have used the monolayer model 2 as local isotherm to determine the adsorption energy distribution (AED) and the pore size distribution (PSD). The adsorption energy values corresponding to active sites of type 1 and type 2 are, respectively, in the intervals [10.5 - 27.5 kJ mol(-1)] and [15 - 25 kJ mol(-1)]. Obtained PSDs indicate that acid blue 9 molecules are adsorbed on macro-pores of Sp LEB-52.