Samples of water with true color of approximately 100 HU, prepared with humic substances of different molecular sizes (filtered through a 0.45 μm membrane with weights in the range of >100 kDa; 100 to 30 kDa; <30 kDa) were studied in order to verify the influence of the apparent molecular size on the performance of coagulation, flocculation and sedimentation of flocs formed using aluminum sulfate as coagulant. To achieve the same degree of color removal, the water samples with higher apparent molecular size fractions required lower dosages of aluminum sulfate (approximately 50% of those necessary for the water prepared with the fraction of the smallest molecules). The results of nuclear magnetic resonance (NMR) and Infrared Spectroscopy (FTIR) studies demonstrate that, under the same conditions, the higher the number of oxygen-containing functional groups, the lower the level of color removal becomes.