One of the fundamental requirements for assessing the suitability of cement-based Stabilization/Solidification as a method for the safe disposal of hazardous materials is the manner in which they will release contaminants over an extended period of time. In this paper the leaching of a synthetic Stabilized/Solidified waste, of composition similar to wastes produced from a commercial process in the United Kingdom are investigated under conditions in which leachant flows through the connected porosity of the sample. The leaching of Zn, Cd, Hg, Pb, Cu, Ni, Fe and Cr has been determined using a modified triaxial cell (flexible wall permeameter). Analysis of unleached solidified waste samples using XRD indicated retardation of conventional cement hydration reactions. The principal crystalline phases observed were ettringite and calcite. Leaching carried out using RO purified water used to simulate a mono-disposal situation, indicated that sufficient buffering capacity remained in the solidified materials to limit the release of all elements except Hg to less than 1% of the total. The leaching of mercury appeared to be related to that of calcium. In this experiment approximately 3600 sample volumes of RO water were passed through the solidified waste. In a second series of experiments (pH=2.88) acetic acid, designed to simulate co-disposal conditions was passed through samples. The acid buffering capacity was found to be insufficient to allow retention of metals after only 50 sample volumes. The order of leaching in this case was found to be Cd > Zn > Cu = Ni > Pb > Hg > Cr > Fe. From this possible retention mechanisms for specific elements are reviewed. In these experiments the total volume of leachate collected was approximately 600 sample volumes. It is believed that this technique has a significant potential for usage in the leach testing of monolithic Stabilized/Solidified waste samples.