Active species including hydroxyl (HO center dot) and sulfate radicals (SO4 center dot-) play important roles in contaminant degradation during the persulfate based in-situ chemical oxidation (ISCO) process. The generation and contribution of active species are critical and can potentially be evaluated using compound specific isotope analysis (CSIA). However, the evaluation of stable isotope fractionation (or isotope enrichment factor s values) for contaminants degraded by individual active species of concern is required but lacking. This study firstly determined the carbon isotope fractionation of trichloroethene (TCE) degradation by SO4 center dot- with chemical probe methods to obtain a values from -6.4 +/- 0.7 to -6.9 +/- 0.5 parts per thousand. The a values were significantly different from those reported for TCE degradation by HO center dot, which could be used to identify the competing TCE degradation by HO center dot and SO4 center dot-. Relying on the observed a values and the extended Rayleigh-type equation, the contributions of SO4 center dot- and HO center dot to TCE degradation were evaluated in per sulfate activated by Fe-0 or Fe(II). This study provides an illuminating idea to determine stable isotope fractionation for contaminant degradation by individual active species, which is crucial for the application of CSIA in relevant environments. (C) 2017 Elsevier B.V. All rights reserved.