The heavy sulfur-rich oils of the Peace River oil sands were formed by severe anaerobic biodegradation. To investigate the impacts of biodegradation on organic sulfur compounds (OSCs) we have measured the distribution and delta S-34 values of alkylated thioaromatics detected in four oils from each of two Peace River wells (A and B). The fluid composition of the Peace River oil sands can be highly complex due to the variable contributions of sources, thermal maturity and secondary alteration. Nevertheless, a previous multimolecular investigation of samples from well B oils (PR2 oil-leg of Marcano et al., 2013, Organic Geochemistry 59,114-132) indicated that the oils had biodegradation levels (BLs) of 5-6, with biodegradation increasing down the oil column. The hydrocarbon composition of oils isolated from well A were biodegraded to a lesser degree than well B (i.e., the presence of isoprenoids and high molecular weight [MW] n-alkanes suggests BLs of similar to 3-4). The concentrations of C-1 -C-3 alkylated benzothiophenes (BTs) and C-1 -C-3 alkylated dibenzothiophenes (DBTs) declined sharply through both wells consistent with increased biodegradation, although other alteration impacts (e.g., water washing) might also contribute. The rates of decrease recorded for selected isomers showed variable responses (e.g., 3&4-mBT > 2-mBT; 4-eDBT > 1,3-dmDBT) which implies different susceptibilities to biodegradation and which may contribute additional useful molecular parameters for evaluating the BL of biodegraded S-rich petroleum. The S-isotopic measurements showed a S-34 enrichment in several alkyl BTs down the oil leg in well B, with their delta S-34 values increasing by up to 6 parts per thousand, consistent with the microbial utilisation of S-32 (n.b., the delta S-34 values of alkyl BTs were not measured in well A oils because of low aromatic sub-fraction concentrations). In contrast, there was little variation in the delta S-34 values of alkyl DBTs down both wells indicating negligible S-isotopic fractionation of these OSCs in the deeper more biodegraded oils, possibly because of their higher MW and resistance to biodegradation. The delta S-34(DBT) (and bulk delta S-34) values of the well B oils were generally slightly enriched than the well A oils probably due to differences in the relative contribution of the major Devonian and Jurassic source rocks. Crown Copyright (C) 2019 Published by Elsevier Ltd. All rights reserved.