Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re-Os date obtained. To minimize the influence of any nonhydrogenous Re and Os, we evaluate analysis of isolated organic matter from the whole rock, together with whole rock analysis using a CrO 3 -H 2 SO 4 digestion medium instead of inverse aqua regia, for a black shale unit of the Exshaw Formation, Canada. This unit previously returned a whole rock Re-Os date of 358+/-10 Ma (Model 3) [Geochim. Cosmochim. Acta (2002)] using inverse aqua regia dissolution. Organic matter isolated from the whole rock matrix using the HF-BF 3 technique [Org. Geochem. 20 (1993) 249] yields scattered data and a Re-Os date of 449+/-220 Ma (Model 3, MSWD=616). The organic matter analyses show similar 1 8 7 Os/ 1 8 8 Os values, but significantly lower 1 8 7 Re/ 1 8 8 Os values in comparison to the whole rock analyses. We show that the Re-Os systematics of organic matter are altered during chemical isolation, and as such we suggest that the HF-BF 3 method should not be used for Re-Os analysis of organic matter. Whole rock Re-Os analysis using a CrO 3 -H 2 SO 4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method, and the Re-Os data identify two distinct initial 1 8 7 Os/ 1 8 8 Os values for the sample set. Separate regressions of these data yield precise dates [366.1+/-9.6, MSWD=2.2 and 363.4+/-5.6 Ma, MSWD=1.6 (Model 3)], which are indistinguishable from the age constraints for this formation (363.4+/-0.4 Ma, U-Pb monazite). Comparison of the Re-Os dates obtained from aqua regia and CrO 3 -H 2 SO 4 methods suggests that the former may contain nonhydrogenous Re and Os, whereas the CrO 3 -H 2 SO 4 method dominantly liberates hydrogenous Re-Os from organic matter, allowing for better stratigraphic age determinations and evaluation of the Os isotope composition of seawater.