Results of pollution flashover voltage measurements and optical observation obtained for high voltage insulators placed in the vertical and the horizontal positions are presented. H was found that the well known difference between flashover voltages measured in tests of insulators placed in these positions was a function of the mean contaminant thickness and the uniformity of layer distribution on the insulators. On the basis of measurements of the ESDD conducted on insulators subjected to prolonged, natural contamination, their self-cleaning, in the vertical and horizontal positions, was determined