Effects of residual contamination that increases contact resistance with mechanical operation was investigated. Metal sealed switches with Ag-Si internally oxidized contacts were used. Contact surface was contaminated in varying degrees by dipping them into trichloroethylene (CHCICCI2) containing 0-3.0 vol% of paraffin lubricant. Contact were mechanically operated more than 106times. Contact resistance increased gradually with operation, so long as the atmosphere contained oxygen. Contaminant was observed concentrated at the contact point. At the same time a method to evaluate surface organic contamination semiquantitatively was studied by Auger electron spectroscopy (AES).