in the present work, we have studied the adsorption of Rhodamine 6G on negative colloidal particles (Silica-Bindzil, Ludox-SM30 and Ludox-HS30) and Calcein on positive colloidal particles (Ludox-CL and Alumina) at very low concentrations by use of fluorescent correlation spectroscopy (FCS). In this relation we have introduced a set of equations which take into account the different quantum yield of the dye molecules and the particles as well as the adsorption of the dye molecules on the sample cell. The concentrations of fluorescent dye were varied between 2 x 10(-9) and 2 x 10(-7) M, and the concentrations of particles were in the range of 0.01-0.03 g/L. The adsorption data fit well with a Langmuir adsorption isotherm. The values of the adsorption constants at pH 5.5-5.7, ionic strength NaCl 1 mM, and room temperature (22 +/- 0.5 degreesC) were in the range (0.2-2.5) x 10(8) L/mol, and the adsorption energy varied between -10 and 13 kJ/mol. The cooperative binding model (generalized Freundlich) was also checked but showed less satisfactory results. This work demonstrates the high performance of the FCS method for investigation of the adsorption equilibrium at solid-liquid interface. In particular the technique does not required any separation step of the adsorbed and nonadsorbed species and is not influenced by the adsorption on the sample cell walls which is often a source of artifacts when working with very low concentrations.