Several experimental methods are proposed to extend the membrane permeation technique to investigate the diffusion of two solvents in a polymer. Either the self or mutual diffusion coefficients of ternary systems can be measured. Two groups of methods are proposed depending on whether tagged solvents are used or not. The use of tagged solvents extends the application of the method to measure the concentration dependency of the mutual diffusion coefficients. This method also allows the measurement of the solubility of two solvents in a polymer in ternary systems.