A theoretical description of surfactant adsorption based on the model of the adsorbed phase being a mixture of single monomers and spherical and globular aggregates of different sizes has been developed. The assumed aggregate shapes were such that the cross-section of each aggregate parallel to the adsorbent surface was a circle with a radius depending on the aggregation number. The aggregates were allowed to move along the surface and all the lateral interactions between them were neglected except the "excluded area" ones. With such assumptions the adsorbed phase could be considered as a 2D fluid of hard disks of various dimensions being under the influence of the potential field due to the adsorbent. Using the scaled particle theory (SPT), the expressions for adsorption isotherm and heat of adsorption were derived and next fitted to the experimental data for three zwitterionic surfactants adsorbed on hydrophilic silica. The detailed expression for the chemical potential of transfer of a surfactant molecule from the bulk phase to the surface aggregate proposed by Nagarajan and co-workers [E. Nagarajan, E. Ruckenstein, Langmuir 7 (1991) 2934; R.A. Johnson, R. Nagarajan, Colloids Surf. A 167 (2000) 21; R.A. Johnson, R. Nagarajan, Colloids Surf. A 167 (2000) 31] was incorporated into the model. On the basis of the obtained results the effect of the surfactant-surfactant and solid-surfactant interactions on the organisation of surfactant molecules at the solid-liquid interface is discussed. (c) 2005 Elsevier B.V. All rights reserved.