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Abstract — The influence of triboelectrification of the contact surfaces on friction coefficient displayed by polymethyl methacrylate (PMMA), and high density polyethylene (HDPE) spheres sliding against polytetrafluoroethylene (PTFE) and steel sheets is discussed. The effect of insulating the sliding surfaces on the friction coefficient is discussed at dry and water as well as salt water wetted sliding conditions. It was found that isolated test specimens showed relatively lower friction coefficient than that observed for the connected ones. This behaviour can be explained on the basis that sliding of PMMA sphere on PTFE surface generated positive charge on the PMMA surface and negative charge on PTFE surface. The lower surface of PTFE sheet would be charged by positive charge which, in condition of connecting the steel sheet by the grounded steel pin, would move to the steel pin and increase the positive charge generated on its surface. In that condition, the electric static charge would increase and consequently the attractive force would increase leading to an increase in friction coefficient. As for isolated test specimens, the electric static charge on the contact would be lower and consequently the attractive force would be lower causing slight decrease in friction coefficient. The same trend was observed for sliding of PE against PTFE and steel. Based on the experimental results, it is recommended to insulate the sliding surfaces in order to decrease friction coefficient