This paper presents the first results of a procedure which allows the determination of the threshold for space charge build-up in polymeric insulation subjected to ac electrical field. Thresholds for space charge accumulation under dc were determined by observing insulation until space charge appeared steadily. In ac the amount of charge accumulated is much smaller than in dc, and the time needed to reach steady conditions can be very long at fields close to threshold. Therefore, ac poling tests were made at different fields and for different times until a chosen value of accumulated charge was reached. Decreasing the test voltage, a limit field was determined below which charge accumulates much more slowly with respect to the linear relation obtained at higher field levels, and its value constitutes an approximate estimate of the searched threshold field. This procedure was applied to specimens of cross-linked polyethylene. A threshold field lower than, but close to, 20 kV/mm was found.