Prescribes methods of test for the evaluation of water treeing in polyethylene (PE) and crosslinked polyethylene (XLPE) compounds, as a means to assess their relative performance under alternating (a.c.) electric stress, in the presence of water. Two methods are described, method I covering the evaluation of insulating materials alone, and method II covering the evaluation of insulating sandwiches consisting of an insulating material in intimate contact with semiconducting screens.