The present study was focused on molecular weight and molecular weight distribution values of pyrolysis oils that could be useful in modeling oil shale thermochemical conversion processes, consistent with principles adopted in advanced coal pyrolysis modeling. Direct pyrolysis Field Ionization Spectrometry (py-FIMS) was applied to evaluate these parameters for volatilizing pyrolysis products with molecular weights greater than 50 daltons. Stepwise heating, with an overall rate of 10 degrees C min (1) up to 710 degrees C, was used. The oil shales studied were Kukersite and Dictyonema oil shales from Estonia and Green River formation oil shale from the United States. Comparison of molecular distributions indicated a decreasing trend in molecular weight parameters (average molecular weight and width of the molecular weight distribution) of oils from higher oil yield oil shales to lower oil yield oil shales. In addition, the molecular weight trends seen among pyrolyzates from this study, under py-FIMS conditions, were compared to published values for pyrolyzates under other pyrolysis conditions and also to published values of the average molecular weight between cross-links for these three oil shales. (C) 2015 Elsevier Ltd. All rights reserved.