This study demonstrates a simple laboratory method to measure diffusion coefficients of reactive and nonreactive solutes in rock blocks. A demonstrative diffusion measurement device consists of a small plastic cell with magnetic stirrers, the porous rock block to be measured, and nonreactive tracer solution. Concentrations of the tracer (as specific conductance) are measured in the cell continuously with data recorded using a PC. The specific conductance versus time data are fitted to an analytical solution that is numerically inverted to estimate the diffusion coefficient. Mathematical development is demonstrated, and a sensitivity analysis shows that diffusion coefficient as low as 10-9 cm2/s can be measured over a test period of 12 days. ¿ 2001 American Geophysical Union