Vacuum Gas Carburizing is an important industrial process used for hardening the steel parts. Although many gases have been used for this process e.g. methane, propane, ethane, acetylene etc. but acetylene is considered more suitable due to certain advantages. The carburizing gas undergoes pyrolysis under low pressure to deliver the carbon on the surface of steel parts. Therefore it is very important to investigate and understand the pyrolysis step in order to optimize the carburizing process. Usually the acetylene contains small amounts of acetone which also undergoes pyrolysis and affects the gas phase composition. The present work investigates the effects of acetone on pyrolysis of acetylene and its possible consequences on the carburizing process. A model has been developed to describe the experimental data under the typical vacuum carburizing conditions using a detailed kinetic mechanism.