The flow-through leaching test is a test method employed to study the leaching behavior of monolithic stabilized/solidified (S/S) hazardous wastes under the condition that the leachate flows through the sample. This method simulates the leaching process of the S/S hazardous waste disposed under a particular landfill condition when the S/S waste is more permeable than its surrounding materials or when the deterioration of the solidified waste form has reached a state that ground water can flow-through the waste via the porosity system of the S/S waste matrix. This paper describes a study on the long-term performance of the cement-based S/S heavy metal wastes using a flow-through leaching test method. Two series of leaching tests with different synthetic heavy metal waste samples were carried out. The S/S samples were made from five types of heavy metals with two kinds of binders. The metals were Pb(2+), Zn(2+), Cu(2+), Ni(2+) (positive ions as nitrates), and Cr(6+) (a negative ion as potassium dichromate), and the binders were type I ordinary portland cement (OPC) and pulverised fuel ash (PFA). The model developed by Godbee and Joy for simulating the leaching behavior was modified to estimate the diffusivity parameter in this study. The results obtained indicate that since the matrix of the solidified waste in a flow-through leaching tests is always being degraded, the values of diffusivities increase continuously during the leaching period. The diffusivity variation range was from 10(-13) to 10(-3)cm(2)/s, and were normally higher than those obtained from other test methods such as ANS 16.1 test and other dynamic leaching tests.