The chemistry in an argon plasma jet, admixed with a small percentage of CH4, C2H2, H2 and mixtures thereof, is investigated by means of residual gas analysis. Polymerization of such hydrocarbon precursors is known to occur when their densities exceed the Ar+ ion density. This paper shows that polymerization also occurs for precursor gas flows far below the initial Ar+ ion flow emanating from the plasma source. This is entirely due to the negative effect of H2 on the Ar+ ion density. Adding 1–2% of H2 to the total argon and hydrocarbon gas flow suffices to initiate polymerization. Although, H2 can be injected directly into the system, fragmentation of the hydrocarbon precursors themselves can likewise supply (part of) the required H2. Polymerization is furthermore enhanced when both precursors are used together. The contribution of C3Hy species to the plasma chemistry will likewise be substantiated.