High-pressure thermogravimetry (TG) with rapid on-line gas chromatography/mass spectrometry (GC/MS) has been used to investigate the effects of different catalysts on decomposition reactions of commingled waste plastics (predominantly PE), high-density polyethylene (HDPE) and mixtures of DECS-6 coal with waste plastics in H-2 at 900 psig. This permits direct evaluation of relative decomposition and residual char amounts as well as yield and composition of the evolved products. Catalysts evaluated for the conversion of waste plastics include solid superacids such as Fe2O3/SO42-, Al2O3/SO42-, Al2O3/SO42- promoted by 0.5% Pt, and ZrO2/SO42- (all added at 10 wt%), as well as a conventional cracking catalyst of SiO2Al2O3 in a 4:1 ratio, a hydrocracking catalyst of NiMo/Al2O3 mixed with SiO2/Al2O3 (both added at 50%), and an HZSM-5 zeolite catalyst (added at 10%). Under these conditions cracking activity for waste plastics reveals the following order: SiO2/Al2O3, HZSM-5 > NiMo/Al2O3 mixed with SiO2Al2O3 > solid superacids. Of the solid superacids studied, the ZrO2/SO42- catalyst possesses the highest cracking activity and the approximate order of cracking activity is ZrO2/SO42- > Al2O3/SO42- > Pt/Al2O3/SO42- > Fe2O3/SO42- > no catalyst. The stronger the cracking catalyst, the lighter the aliphatic products and the more abundant the isomeric constituents. Similar results are found for HDPE with these catalysts. For co-processing of coal with commingled waste plastic the HZSM-5 zeolite catalyst shows the most promising results by increasing the rate of the decomposition reactions at 420 degrees C nearly tenfold. Hydrocracking catalysts, such as NiMo/Al2O3 mixed with SiO2/Al2O3, show potential promise for co-processing of coal with commingled waste plastic due to their combined hydrogenation and cracking ability. By contrast, a superacid such as ZrO2/SO42- or a cracking catalyst such as SiO2/Al2O3 appears to have little effect on the decomposition rate of the mixture. To what extent these findings are influenced by transport limitations (e.g. due to incomplete mixing or degree of crystallinity) and/or catalyst pretreatments is being studied further.