SUMMARYEleven horizons of acidic soils in mid‐Wales developed from Lower Palaeozoic sedimentary rocks were examined. Selective extraction of Al and Si provided evidence against the occurrence of significant quantities of poorly ordered Al‐silicates. Fe0was weakly correlated with Al0, but very closely correlated with Al0minus Al extracted by cold 5% Na2CO3, implying that poorly ordered Al occurs in part as a substituent in Fe oxide and in part in a form unassociated with Fe oxide. Support for this was obtained by analysis of oxide fractions concentrated from aqueous suspensions by sequential ultracentrifugation and through the examination of synthetic Al‐substituted Fe oxides. Fe oxide containing Al substituted at an almost constant level was the dominant constituent of the poorly ordered fraction in four of the five Bs horizons examined. The occurrence of Al in this form is an important mechanism by which Al is retained in aerobic but highly acidic Bs hor