A new on-line, nonintrusive monitoring system, able to observe the aging of an ac machine winding insulation is described. The principle of this system consists of detecting small variations of the turn-to-turn capacitances due to the dielectric aging. The authors have performed experiments on accelerated insulation aging as well as on a system able to measure machine winding high-frequency resonance, which depends on turn-to-turn capacitance. First, twisted pairs of magnet wires were submitted to thermal and electrical stresses, in order to establish a connection between the turn-to-turn capacitance with the breakdown voltage and the probability of failure. Then, the changes in the resonance frequency of a simplified stator winding, the "motorette," were measured. Measurements of the currents or of the-magnetic field in the vicinity of the machine can be made. The analysis of the experimental data yields information on the turn-to-turn capacitance variations, hence on the insulation aging.