Results are reported for ellipsometric measurements of hydrophobized monodisperse silica particles, with a diameter of about 25 nm, spread at the toluene-water interface. Theoretical values for the ellipsometric parameters are derived by treating the particles as a core-shell model and performing integrations of the refractive index profile through the interface using Drude's equations. With justifiable choices of the fixed parameters for the system, the agreement is good between measured and calculated values for the ellipsometric parameter Delta as a function of the amount of silica particles added to the interface. However, the results at high particle concentration at the interface are consistent either with coverage greater than a close-packed monolayer or with a monolayer with corrugations whose amplitude is less than the radius of the particles. The results show that this is not a suitable method for the determination of the contact angle of the particles at the oil-water interface.