A new method is described for the determination of the isotopic composition and chemistry of groundwater in aquitards. The effective porosities for each component can be determined concurrently. The method is based on diffusive exchange of water and solutes between groundwater in a cylindrical core of aquitard material and water in a reservoir placed along the axis of the cylinder. The method was tested for 2H, Cl, and SO4 in an aquitard consisting of clay-rich glacial till. The Δ2H in aquitard core samples as determined by radial diffusion agreed closely with Δ2H in groundwater obtained from corresponding piezometers installed in the aquitard. For the concentrations of Cl and SO4 the agreement was less close but still satisfactory. The effective porosities were equal to the total porosities for 2H and were less than the total porosities for Cl and SO4 ions. ¿ American Geophysical Union 1996