The total recovery of natural gas from subsea wells can be significantly increased with a compressor installed near the wellheads. The compressor is powered by a high-speed induction motor integrated in the same casing. The process gas flows through the motor and acts as a cooling medium. The insulation system of the motor is in direct contact with the gas and must be resistant to it. The gas mixture contains hydrocarbons, water, and monoethylene glycol. The effects of the gas mixture and its individual components on the properties of a high-voltage machine mainwall insulation consisting of mica, glass, and epoxy are obtained by experimental tests with raw natural gas at accelerated conditions. The tests at high pressures and temperatures indicate that heavy hydrocarbon compounds cause similar effects to plasticizers inside the bisphenol A epoxy resin, but such compounds do not penetrate easily into epoxy novolac resin. The plasticizing effect is seen as increased weight and volume, decreased mechanical strength and E-modulus, and reduced glass transition temperature. The polymers did not decompose chemically. The mainwall insulation is vulnerable to delamination, which is initiated by the detachment of glass strains and epoxy resin. Water causes dielectric loss peak at very low frequencies, while the heavy hydrocarbons produce a loss peak in higher frequency range. © 2012 IEEE.