Samples from the various horizons of five soil profiles representative of diverse types widely occurring in north-east Scotland were intensively examined, after separation into particle-size fractions, by appropriate instrumental and chemical techniques including optical examination, X-ray diffraction, electron microscopy and diffraction, infra-red absorption spectroscopy, thermal analysis procedures, chemical analysis and selective chemical dissolution. The results obtained are presented and assessed with respect to quantitative estimation of the mixed SiO2-Al2O3-Fe2O3 gel system occurring in these soils. At best results can be only semiquantitative, but interesting trends are observable in each profile and marked differences are noted between profiles on different parent materials, although this effect is somewhat obscured under impeded drainage conditions. The maturity of the soil is also a significant factor. Results in general can be explained in terms of current concepts on pedogenesis.