This study investigates the dithiothreitol (DTT)-based oxidative potential (OP) and corresponding hydroxyl radical (& BULL;OH) generations capacity of PM2.5 (particulate matter with aerodynamic diameter & LE; 2.5 mu m) over a high-altitude site, Shillong (25.7 N, 91.9 E; 1064 m above mean sea level), located in the northeastern Himalaya. Measured OP and & BULL;OH are reported in two units: per m3 of filtered air (OPV and OHV) or per unit mass of PM2.5 in mu g (OPM and OHM). Based on the characteristic ratios of chemical species in different sources, PM2.5 were classified into three source categories: Biomass Burning (BB) (N = 10), Secondary Organic Aerosols (SOA) (N = 12), and Mixed (N = 12). Consistently higher OP and & BULL;OH generation per m3 of air (OPV: 6.2 & PLUSMN; 1.0 (average & PLUSMN; SD, 1 & USigma;) nmol DTT min- 1 m- 3, and OHV: 1.2 & PLUSMN; 0.31 nmol & BULL;OH m- 3) in all BB-dominated PM2.5 samples indicate relatively hazardous exposure dose from BB emissions among all sources. Characteristic OC/EC, WSOC/ OC, and nss-K+/EC ratios evoked that SOA from biogenic precursors was a significant source for the observed OP and & BULL;OH in SOA-categorized samples. Distinctly different chemical composition of PM2.5 over Shillong (about 55% composed of organic matter) makes it relatively more (2-4 times) intrinsically redox-active compare to a reported study over another high-altitude site in a semi-arid region of India. Despite-1.7 times lower PM2.5 mass over Shillong (characterized by more aged PM2.5) compare to Patiala (30.3N, 76.4E; 250m amsl, a semi-urban city, characterized by fresher PM2.5 from mixed sources), the observed OPV over Shillong was about 1.2 times higher, and the OPM was about 2 times higher. It is attributed to the effect of atmospheric processing on particle's redox-activity. While examining the relationship of OPV and OHV with chemical species, it was observed that & BULL;OH generation capacity was more influenced by WSOC species' origin (primary or secondary) compare to DTT consumption. Such an observation restricts the practicality of particle-induced OP to be considered as the only metric for aerosol particle toxicity, and suggests also to consider simultaneous measurements of & BULL;OH generations. For the BB-dominated category, OC-normalized OP (OPOC) was correlated strongly with the fractions of N -containing water-soluble organic aerosols (i.e., a fraction of WSON and a fraction of familyCHO1N). Corre-sponding OC/EC, nss-K+/EC and nss-SO2-4/EC ratios hint that coal combustion, in addition to BB, could be a significant source of redox-active WSON species in these samples. Such studies are important for identifying redox-active aerosol species and developing appropriate mitigation policies for healthy air.