The solubility of pure 1,3 butadiene and methyl propene and their mixtures in pure n-methyl-2-pyrrolidone and in its aqueous solutions was measured at several temperatures and pressures. For this purpose, experimental methodologies were established in order to evaluate thermodynamic equilibrium variables. A simple weighing method was used to obtain the experimental data. From the experimental results, the parameters of conventional excess Gibbs free energy models, Wilson, NRTL and UNIQUAC, were determined through the Maximum Likelihood method.