A self-assembled monolayer of decyltrichlorosilane (Cl3Si(CH2)(9)CH3) supported on elastomeric polydimethylsiloxane (PDMS) was used as a model system to study the adsorption of a nonionic surfactant [H(CH2)(12)(OCH2CH2)(7)OH] at the solid-liquid interface. The deformation produced on contacting a semispherical lens of the elastomer with a flat sheet under water varied systematically in response to the amount of surfactant added to the aqueous phase. The JKR theory of contact deformation in conjunction with the Gibbs’ theory of interfacial thermodynamics yielded the required interfacial tension and surface excess quantities at solid-liquid interface.