A novel and sensitive approach for the accurate determination of Sb in natural waters is described using photochemical vapor generation (PVG) coupled with inductively coupled plasma mass spectrometry (ICPMS) for detection. Utilizing a unique flow-through photochemical reactor capable of subjecting the samples to deep UV (185 nm) radiation, generation efficiency was found to be independent of whether Sb(III), Sb(V) or organometallic species [trimethyltantimony(V)dibromine, TMSb(V)], eliminating the shortcoming of Sb species depended sensitivity encountered during direct solution nebulization by ICPMS. Furthermore, the potentially severe matrix effect from seawater was efficiently eliminated by using a mixture of 5% (v/v) formic and 15% acetic acids (v/v) as the photochemical reductant, making direct determination of Sb in seawater feasible. The proposed method provides a 15-fold improvement in sensitivity over direct solution nebulization. A method detection limit of 0.0006 ng g-1 based on external calibration was obtained (0.0002 ng g-1 for isotope dilution), yielding a 15-fold improvement over that for direct solution nebulization. Accuracy is demonstrated by analysis of two water Certified Reference Materials (CRMs, e. g., SLRS-6 and NIST 1640a) with satisfying results. In addition, spike recoveries of 100.6 ± 5.5% and 100.8 ± 3.8% (SD, n=3) were obtained for NASS-6 and CASS-5 seawater CRMs, respectively, since no certified values for Sb has been established for these materials. The performance of several calibration strategies, including double isotope dilution (ID), multiple and single point gravimetric standard additions with internal standardization as well as multiple and single point gravimetric standard additions alone was examined. High precision of determination of Sb in four natural water samples (0.51 to 1.4%) was realized based on ID calibration, whereas one point gravimetric standard addition calibration with internal standardization provided precisions of 1.6 and 3.3% at 0.22 and 0.44 ng g-1 levels, respectively, in seawater.