Measurements of the resistivity of good insulators were carried out by measuring the loss of charge on a condenser. A knowledge of the dimensions of the insulator is not required. It was found that the insulation resistances of amber, alkathene, distrene and Perspex increase with time over a period of weeks, the final values being considerably higher than previously published figures. There is no significant difference between the values for the first three materials. The leakage time necessary for reasonable accuracy is indicated.