Vermiculitic and kaolinitic materials were used to determine the effects of these clay minerals on the phosphorus fixing capacity of two sandy soils, a Podzolic and a Latosol, both occuring in the Piracicaba municipality. Quantities varying from 1.25% to 25% (weight basis) were thoroughly mixed with samples taken from the Ap horizon of the two soils. Two groups of treatments thus prepared received an application of soluble phosphorus at the rate of 50 and 100 ppm, and were incubaded for two different periods, 10 and 20 days. The clayed materials and soils were analysed as to their mineralogical characteristics. The chemical characteristics and particle size distribution of the samples were also determined. The results obtained were statistically analysed. The following condusions were drawn: 1. The vermiculitic material and the Podzolic soil had a greater phosphorus fixing capacity than the kaolinitic material and the Latosol soil, respectively. 2. Percentage of phosphorus fixed by the soils increased with the percentage of vermiculite approaching the fixing capacity of the pure clay-mineral. This effect was attibuted to the nature of the clay (vermiculite) and to a high magnesium concentration in a pH range favorable to the precipitation of phospho-magnesium compounds. 3. Amount and percentage of fixed phosphorus increased with time of incubation for all treatments, but approximately 90% or more of total P fixation occurred after 10 days, indicating that the phosphorus fixing processes stabilized between 10 and 20 days of incubation. 4. For a given incubation period (10 or 20 days), the amount of phosphorus fixed (ppm) increased with rate of P added, but the % of phosphorus fixed decreased, indicating that the samples were probably saturated with the phosphate ion.