Complex formation between kaolinite and the clay fractions of Ultisols and humic-fulvic acids extracted from a Paleudult was investigated by Sephadex gel filtration and infrared spectroscopy.Humic acid remaining in solution after complex formation exhibited infrared spectrograms with increased absorption in the 1050, 1400, and 1600 cm−1regions for Si-O and carboxyl vibrations, respectively, coupled with the appearance of new peaks at 940–840 cm−1for O-A1-OH vibrations. Gel filtration of the humic acid above resulted in the separation of HA/I (high molecular weight) with strong infrared Si-O absorption, and of HA/II (low molecular weight) with spectrograms revealing the O-A1-OH absorption. Admixture with clay particles was eliminated by Sephadex, since clay was not eluted through the column. Infrared analysis of the clay material before and after reaction with humic acid, showed spectrograms indicating the presence of possible structural changes. After reaction with humic acid, kaolinite and soil clays had spectrograms with decreased absorption for Si-O vibrations. These observations suggested the possibility of a gradual decomposition of the Si-O-Si linkages in the clay structure by acidolytic attack of humic acid. Increased amounts of Si and Al contents found in the remaining supernatant humic acid solution were supporting evidence for the release of these elements by clay.