A first-of-its-kind, gas-tight MD system was designed to provide insight into the dynamic transport of semi volatile contaminants over time by closing the mass balance for each contaminant. Testing with non-volatile ions was used to confirm the integrity of the test system and sampling procedures. Non-volatile contaminants had consistently high rejection rates (>= 99%) for all analytes tested. For the most hydrophobic semi-volatile contaminants, sorption to the membrane and polymeric components within the system played a significant role in their fate. Less hydrophobic semi-volatile contaminants reached a pseudo-equilibrium governed largely by contaminant volatility, as predicted by Henry's Law. Rejection of semi-volatile nitrosamines varied from -96 to 88%, demonstrating significant variation in behavior even within the same contaminant class. Results suggest that transport of semi-volatile contaminants in MD was consistent with a pseudo-equilibrium partitioning process and was highly correlated with contaminant volatility (R-2 = 0.934 for the combined data set). These results have implications for treatment of complex source waters where an understanding of the transport of volatile components is needed even if treatment objectives are focused on nonvolatile components.