A differentially pumped quadrupole mass spectrometer was used to analyze the vapor species present in the deposition chamber during the vapor deposition of poly-para-xylylene (parylene) thin films. The partial pressure in the deposition chamber of volatile contamination originating in the dimer source, di-para-xylylene, was determined. The solvents xylene and acetone, as well as water, were determined to be the main contaminants. Fragmentation patterns of the monomer and dimer were gathered and the composition of the main peaks in each pattern was determined. The largest peak was 104 atomic units (amu) in the monomer pattern and 103 amu in the dimer pattern. The quantitative conversion of dimer to monomer was studied as a function of the pyrolysis furnace temperature from 345 to 665 degrees C for pyrolysis tube liners of quartz, copper, and nickel. It was found that conversion begins at around 385 degrees C, and by 565 degrees C 100% of the dimer flowing through the pyrolysis zone is converted (or cracked) into the monomer.