PDP is a very interesting phenomenon and presents a significant problem to the fuel control and the safety in Tokamak devices. Systematic experimental studies on the PDP mechanism using an ECR discharge plasma have been carried out to investigate the role of the neutrals and to estimate the isotope effects occurred in PDP for H, D, and T. The maximum isotope effects observed are 1.42 for H/D, and 1.1 for a D/T mixed plasma in the case of SUS 304 test membrane. The magnitudes of the isotope effects between the three hydrogen isotopes are satisfactorily explained by the role of neutrals in PDP.