SummaryStudies of the dispersion of compound soil particles by sonic vibration and cation‐exchange resin techniques indicate that the difficultly dispersible particles in mineral soils of high base status are microaggregates (<250μ diam) consisting largely of clay and humified organic material linked by polyvalent metals. The inter‐particle bonds in these microaggregates can be disrupted by application of mechanical energy (sonic vibration or prolonged shaking with water), the amount of energy required for dispersion of clay‐size mineral material being reduced by treatments that weaken or destroy these bonds (e.g. treatments leading to replacement of polyvalent metals by monovalent metals or to destruction of organic matter). A theory is proposed depicting microaggregate formation as a solid‐phase reaction involving linkage of electrically neutral clay mineral and organic matter particles by polyvalent metals on exchange sites, and microaggregate disruption by sonic vibration as a reversal of this reaction. Experiments to evaluate this theory are d