A previously published paper presented product composition results in peak area percentage following pyrolysis of waste high-density polyethylene experiments in a two-stage micropyrolysis reactor. Pyrolysis experiments were performed at 625, 650, and 675 degrees C with vapor residence times (VRTs) varying from 1.4 to 5.6 s (Gracida-Alvarez, U. R.; et al. Ind. Eng. Chem. Res. 2018, 57, 1912-1923). This brief communication discusses the methods used to convert gas chromatography/mass spectrometry (GC/MS) peak areas to mass results and provides a brief analysis of product mass compositions. Product mass composition provides valuable information for future work and comparison to studies reporting product yields in mass units. The mass fractions of gas products and aromatics were found to increase with both temperature and VRT, while liquid and wax fractions declined.