Abiotic and biotic reductive dechlorination with chlorinated ethene dense non-aqueous-phase liquid (DNAPL) source zones can lead to significant fluxes of complete and incomplete transformation products. Accurate assessment of in situ rates of transformation and the potential for product sequestration requires knowledge of the distribution of these products among the solid, aqueous, and organic liquid phases present within the source zone. Here we consider the fluid-fluid partitioning of two of the most common incomplete transformation products, cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC). The distributions of cis-DCE and VC between the aqueous phase and tetrachloroethene (PCE) and trichloroethene (TCE) DNAPLs, respectively, were quantified at 22 °C for the environmentally relevant, dilute range. The results suggest that partition coefficients (concentration basis) for VC and cis-DCE are 70 ± 1 L(aq)/L(TCE DNAPL) and 105 ± 1 L(aq)/L(PCE DNAPL,) respectively. VC partitioning data (in the dilute region) were reasonably approximated using the Raoult's law analogy for liquid-liquid equilibrium. In contrast, data for the partitioning of cis-DCE were well described only when well-parametrized models for the excess Gibbs free energy were employed. In addition, available vapor-liquid and liquid-liquid data were employed with our measurements to assess the temperature dependence of the cis-DCE and VC partition coefficients. Overall, the results suggest that there is a strong thermodynamic driving force for the reversible sequestration of cis-DC and VC within DNAPL source zones. Implications of this partitioning include retardation during transport and underestimation of the transformation rates observed through analysis of aqueous-phase samples.