Since the earliest days of this field there has been an interest in correlating the structure of plasma polymer (PP) coatings with deposition parameters, most particularly with energy input per monomer molecule, Em. Both of our laboratories have developed methods for measuring Em (or somewhat equivalent, the apparent activation energy, Ea) in low- (LP) and atmospheric-pressure (AP) electrical discharge plasmas. We recently proposed a new parameter, energy conversion efficiency (ECE), which for the first time permits direct comparison of LP and AP experiments. Here, we report the case of small hydrocarbons, namely acetylene, ethylene and methane. "Critical" Em (or Ea) values that demarcate ECE regimes separating different reaction mechanisms are found to agree remarkably well, and to correlate with specific reaction mechanisms, including dissociation, recombination, gas-phase oligomerization, and surface processes.