The content of water-dispersible colloids (WDC) has a major impact on soil functions and structural stability. In addition, the presence of mobile colloids may increase the risk of colloid-facilitated transport of strongly sorbing environmental contaminants. The WDC content was measured in 39 soils, using laser diffraction, by agitating the samples using a wet-dispersion unit. This approach eliminated the need for long sedimentation times required by the more classical end-over-end shaking approach and provided information about the time-dependent release of WDC. The total clay content of the soils ranged from 0.1 to 0.44 kg kg−1. The WDC content was measured on air-dry and moist 1- to 2-mm aggregates. The WDC content at a reference time was highly correlated to the total clay content (r > 0.91, P < 0.001) for all soils. Only for two sites was the WDC content correlated to the content of clay not associated with organic C (r > 0.89, P < 0.001), calculated as a function of total organic C and total clay. The colloid release rates were highly correlated with the total clay content (r > 0.84, P < 0.001). The WDC content in moist aggregates measured using laser diffraction was correlated with the WDC content measured using a more classical end-over-end method (r > 0.89, P < 0.05) and in 100-cm3 undisturbed soil cores (r > 0.89, P < 0.05). The universal correlation between the contents of WDC and total clay could be highly useful in risk assessments of colloids and colloid-facilitated transport of environmental contaminants.