The growth kinetics of films formed from the thermal decomposition of small, volatile chlorinated hydrocarbons on iron have been previously measured at relatively high pressures (a few Torr) using a microbalance. These results were used to model the tribological behavior of these molecules as extreme-pressure lubricant additives. Similar chemistry is examined on clean iron prepared in ultrahigh vacuum using a de molecular beam created using a directional dosing source where the beam is incident on the clean sample and the gas-phase products of the reaction are detected mass spectroscopically. The temperature-and pressure-dependence of the decomposition rate of both methylene chloride and chloroform are found to agree well with the kinetics measured in the microbalance. In addition, the nature of the reactive film, which consists of FeCl2 and incorporates carbon, is found to be identical in both regimes suggesting that film growth kinetics can be successfully measured in ultrahigh vacuum. Finally, the only gas-phase reaction product detected in either case was hydrogen, indicating that both reactants thermally decompose to form FeCl2, deposit carbon, and evolve hydrogen.