The particle-size distribution of illite and iron oxide-precipitated illite was examined as a function of pH by means of the sedimentation balance. Due to the precipitation of 5% iron oxides on the illite surfaces, aggregation occurred over a pH range of 3 to 10, and the degree of aggregation was greater for higher pH values. The aggregation was explained by the decrease of repulsion between face surfaces of illite due to the partial neutralization of the negative charge of illite surfaces, based on the electron micrographs and zeta potential measurement.