The sensitivity of the pyrene monomer emission spectrum to the polarity of a medium has been exploited to probe the adsorbed layer of a series of polyoxyethylated alkyl phenols on silica surface. Both the hydrophobicity and hydrophilicity of the surfactants are found to control the adsorption of the surfactants at the silica-water interface. In a series of surfactants having the same hydrophobic chain length but with different hydrophilic chain lengths, the presence of a smaller number of hydrophilic oxyethylene units leads to a conspicuous increase in adsorption density in the premicellar region compared to surfactants having a larger number of oxyethylene units. This has been attributed to the formation of small aggregates (hemimicelles) on the silica surface in the former. The formation of the aggregates gets support from the emission characteristics of pyrene. The number of surfactants constituting different aggregates has been determined. (C) 2003 Elsevier Inc. All rights reserved.