This guide is intended to apply to silicone-immersed transformers in which the silicone fluid was the fluid supplied when the transformer was originally manufactured. This guide also addresses the following: a) The theory of combustible gas generation in a silicone-immersed transformer; b) Recommended procedures for sampling and analysis; c) The interpretation of Gas Analysis; d) Recommended actions based on the interpretation of the results and e) A bibliography of related literature.