Method for removing tar-forming hydrocarbons from an effluent gas mixtures from Chemical Vapor Deposition or Chemical Vapor Infiltration processes. Method includes passing at elevated temperature effluent gas mixture containing hydrogen, methane, and high molecular weight hydrocarbons through a bed that contains iron pellets, thereby decomposing tar-forming high molecular weight hydrocarbons in the effluent gas mixture. Apparatus including a de-tarring vessel (5) having a packed bed (7, 8, 9) of iron or iron oxide pellets (1) resting over a perforated distributor plate (2) and having an exhaust port (12), said de-tarring vessel being operatively linked via an exhaust port (6) to a CVI or CVD reactor vessel.