Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM2.5 sulfate (Delta O-17(SO42-)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Through-out the sampling campaign, the 12-hourly averaged PM2.5 concentrations ranged from 16 to 323 mu g m(-3) with a mean of (141 +/- 88 (1 sigma)) mu g m(-3), with SO42- representing 8-25% of PM2.5 mass. The observed Delta O-17(SO42-) varied from 0.1 to 1.6% with a mean of (0.9 +/- 0.3) %. Delta O-17(SO42-) increased with PM2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM2.5 >= 75 mu g m(-3)) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III-V, heterogeneous sulfate production (Phet) was estimated to contribute 41-54% to total sulfate formation with a mean of (48 +/- 5) %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggested S(IV) (= SO2 center dot H2O + HSO3- + SO32-) oxidation by H2O2 in aerosol water accounted for 5-13% of P-het. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Delta O-17(SO42-). Heterogeneous sulfate production via S(IV) oxidation by O-3 was estimated to contribute 21-22% of P-het on average. Heterogeneous sulfate production pathways that result in zero-Delta O-17(SO42-), such as S(IV) oxidation by NO2 in aerosol water andor by O-2 via a radical chain mechanism, contributed the remaining 66-73% of P-het. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 +/- 0.1 or 4.7 +/- 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O-2. Our local atmospheric conditions-based calculations suggest sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O-2 can be the dominant pathway providing that highly acidic aerosols (pH <= 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Delta O-17(SO42-) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.