Abstract Previous investigators have noted that small particles at a fluid interface may exhibit a special tendency to aggregate. Colloidal dispersions which are stable in the bulk often form a coagulum at the air—liquid surface. Such surface aggregation is an important factor in operations that require clean or controlled interfaces. The present work seeks to describe particle adsorption and aggregation at the air—water interface in quiescent systems. A model is proposed for two-dimensional rapid aggregation as well as for slow surface aggregation dependent upon modified van der Waals and electrostatic interactions between particles at the surface. Experimental results from direct observation by darkfield microscopy of particle adsorption and surface aggregation are reported and found to be in qualitative agreement with the model predictions.