An extensive analysis of organics and inorganics present in leachate from landfills located in different regions of the United States was performed to evaluate methods for treating leachates. Bench-scale performance data and evaluation of leachate treatment methods are reported in Volume II. Gross properties of the leachate quality varied greatly between different landfills. Membrane ultrafiltration, gelpermeation chromatography, and specific organic analysis were used to separate different molecular weight fractions and to determine the main classes of organics and associated functional groups. The majority of the organics were able to permeate a 500 MW UF membrane indicative of their low molecular weight. Membrane fractionation and organic analysis of leachate samples collected from different landfills generally showed a decrease of the free volatile fatty acid fraction with increasing age of the fill. Biological degradation studies showed the sequential removal of different classes of organics. Four sequential phases were recognized: (1) removal of high molecular weight humic carbohydrate-like organics; (2) removal of free volatile fatty acids; (3) removal of facterially excreted carbonyl compounds and amino acids; (4) removal of high molecular weight carbohydrates produced during the third phase. Membrane fractionation analysis showed that the majority of the metals permeated the 500 MW UF membrane, indicating that chelation of most metals by refractory organics in leachate plays a minor role in metal attenuation processes; an exception was iron, most of which was associated with the 100,000 MW UF retentate.