In this paper the percolation theory is employed to study the insulator-to-conductor transition in sulphonated poly(phenylene oxide) (SPPO) polymer membranes. The membranes with different sulphonation degree were prepared and infrared characterized. The transition threshold was calculated by the experimental data of membrane conductivities and the coordination number was thus estimated. The functional group -SO3H distribution in the membrane was evaluated in accordance with the calculations and showed a non-random dispersion on the microscopic scale.