The purpose of this article is to characterize and compare the residual COD of raw landfill leachate and its low and high molecular weight fractions before and after aerobic degradation process. The low and high molecular weight fractions (<10 kDa and >10 kDa, respectively) were obtained by the use of an ultrafiltration cell. Samples of the fractions with molecular weights 10 kDa, as well as the raw leachate, were characterized in terms of COD, protein, carbohydrate and lipid concentration and by biodegradability test. The compound identification of all samples was carried out using gas chromatography coupled with mass spectrometry (GC/MS). The results show that the landfill leachate studied is constituted of approximately 60% of compounds with molecular weight <10 kDa. Approximately 80% of the compounds identified in the leachate had been degraded. This is an indication that most of the compounds that constitute the significant fraction of residual COD correspond to intermediate products and products of condensation of affluent compounds or had been generated during the degradation (SMP). Similar compounds were identified in all effluents of the degradation assay, suggesting the presence of SMP. These compounds, predominantly aliphatic and esters, are characterized by high molecular weight and probable refractory nature.