This article presents a new method to construct binary co-adsorption isotherms and to obtain the thermodynamic parameters entering in their description. More precisely, we show how activity coefficients may be extracted from peak chromatography experiments in binary mixtures of co-adsorbed components. These activity coefficients are defined in the framework of the so-called non-ideal adsorbed solution theory. Explicit analytical expressions are established relating the overall adsorption coefficient (and thus the retention time of the peak), to the pure component adsorption isotherms, to the mixture composition, and to the activity coefficients. Owing to thermodynamic constraints (Gibbs-Duhem), the measurement of retention times furnishes enough information, in this framework, to retrieve the activity coefficients. The approach is independent of any model for the adsorption isotherms, but requires a model for the free energy of mixing, or for the dependence of activity coefficients on composition.Experimental examples illustrating the procedure are given for co-adsorption of the binary mixtures CH4/C2H6 (nearly ideal) and C2H6/CO2 (strongly non-ideal) on 5A zeolite.