A novel method for measuring local permeabilities of gas or vapor in polymers was proposed and developed. The method utilizes a thin reactive layer of potassium prepared on one side of the examined polymer film. Molecules of an appropriate gas (oxygen, water vapor) diffusing through the film corrode the reactive layer to various extent at various places, proportionally to the amount of the gas passed. A "permeation map" is created which can be visualized with optical microscopy. Compatibilized polystyrene/polypropylene blend was examined with this method. The permeability map pattern obtained was in accord with the morphology of that system. The lateral resolution attained was approximately 2 mum.