The effect of a number of synthetic variables on the affinities of resorcinol-formaldehyde-iminodiacetic acid resins for137Cs and90Sr was determined. Porosity was introduced into the resins by inclusion of CaCO3 as a solid template during the synthesis. Among the variables examined were reaction temperature, reactant ratio, choice of basic catalyst, and amount of added template. Only reaction temperature was found to have a clearly defined influence on affinity. Cesium-137 affinity increased with increasing reaction temperature. Affinity for90Sr was independent of reaction temperature below reflux temperature, but dropped drastically for resins synthesized at reflux. These results are explained mechanistically. The reproducibility of the resin synthesis is also examined.