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Non-linear losses in heterogeneous iusulation (e.g. impregnated paper) are often attributed to ions oscillating between insulating paper walls without penetrating or even adhering on them. To investigate this problem, we studied the physico-chemical properties of capacitor paper and of other insulating solids having the same blocking behavior. We have investigated the transport of electrolytes across paper impregnated with organic solvents due to an applied electric field or to a pressure gradient, the adsorption of electrolyte, and the net charge carried by paper by means of electrophoresis. It turns out that in the organic liquids tested, paper behaves like a semi-permeable barrier. Synthetic polymers also exhibit the blocking effect when they are made selective by grafted weak acid groups. We demonstrate that the ion blocking effect is of a physical and chemical nature and not of a mechanical one. The chemical nature of paper and impregnant modifies the amount of absorbed electrolyte and above all changes the sign of the net charge on the paper; on the other hand, the pore diameter modifies the concentration value for which selectivity no longer operates.