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In this study the effect of borax existence on tracking and erosion resistance of polyesters has been investigated by means of defining contact angle and performing SEM analysis. Samples with different borax concentrations were tested under 4 kV AC voltage in an inclined plane tracking test apparatus prepared according to ASTM D2303 standard. The results and SEM analysis indicate that addition of borax mineral to test specimens prevents the formation of free carbon over the surface of the composite samples and hence the specimens are mostly damaged by the erosion of the surface. Also by increasing the borax ratio in polyester samples, the contact angle increases and these results also supports the change in surface behavior for erosion rather than tracking.