Contaminant mass flux emanating from a source can serve as a performance metric for technologies aimed at source-zone management. The link between source zone remediation and contaminant plume response is under investigation, and a key to this link is the mass flux leaving the source zone. Unfortunately, mass flux from source zones has been measured only rarely, and long-term monitoring following source zone remediation typically is inadequate. The Sages dry cleaner site in Jacksonville, FL, is an exception. A cosolvent flood was conducted at the site in 1998, and extensive monitoring continues. The monitoring network includes multilevel samplers in the source zone and along a transect located downgradient. This transect has provided data needed to assess contaminant mass flux during the four years following the cosolvent flood. Tetrachloroethene (PCE) and its degradation byproducts were monitored along with residual ethanol from the cosolvent flood. The changes in the mass flux associated with these constituents, plus mass removal assessed during the cosolvent flood, provide insight on the relationship between mass removal and mass flux at this site. The authors discuss issues associated with estimating mass flux using multilevel sampler transects and alternative methods for flux measurement.