Geosynthetic clay liners (GCLs) containing polymer-modified bentonite (PMB) are increasingly being applied as components of hydraulic barrier systems in lieu of conventional sodium bentonite (Na-B) GCLs for improved performance in the presence of chemically aggressive leachates. A comprehensive suite of laboratory experiments was conducted to evaluate the physio-chemical properties of various types of polymers and PMBs. The overall objective of the testing series has been to develop more fundamental understanding of the physical and chemical mechanisms by which PMBs maintain low hydraulic conductivity to aggressive permeant solutions, the limits of this performance, and alternative tests methods by which this performance may be assessed. Conventional index tests including free swell, liquid limit, and fluid loss are not reliable for screening hydraulic performance and chemical compatibility of PMB-GCLs. New index methods for assessing chemical compatibility and hydraulic conductivity of PMB-GCLs are needed. Improved basic understanding of the basic physical and chemical mechanisms that govern liquid flow and solute transport for PMBs is required. This study has five main objectives: 1. Quantify the pore structure of polymer modified bentonite in the range of meso- and micro-pores to clarify where and in what form the polymer fraction in PMB is located and to evaluate corresponding pore size distribution (PSD) of PMB over a wide range. 2. Develop and evaluate new index tests for potential use as preliminary screening tools to assess chemical compatibility and hydraulic conductivity of PMB-GCLs. 3. Assess polymer conformation (shape and structure) and polymer hydrogel development in different aqueous chemistries and relate polymer conformation and quality of polymer gel to the hydraulic behavior of PMB. 4. Evaluate the effect of hydraulic gradient on hydraulic conductivity of PMB-GCLs and identify corresponding implications of polymer elution/clogging, which is potentially affected by hydraulic gradient, to the hydraulic conductivity of PMB-GCLs. 5. Evaluate the effects of anion type on index properties of polymers and polymer bentonite mixtures and on the hydraulic conductivity of PMB GCLs.