This International Standard is a guide to the maintenance of synthetic organic esters, originally complying with the requirements of IEC 1099, in transformers with rated voltages up to 35 kV. It is intended to assist the equipment operator in assessing the quality of the liquid during use in the equipment and maintaining it in a serviceable condition. Methods for reconditioning ester transformer liquid are outlined. This standard deals only with ester liquid used in equipment where liquid sampling is practicable and which has not previously been filled with any other liquid.