The monolayer behavior of different end-functionalized poly(dimethyl siloxane)s (PDMSs) and a block copolymer, poly(dimethyl siloxane-co-ethylene oxide) (PDMS-co-EO) spread on an oligomeric poly(ethylene glycol (PEG) substrates, were studied by means of electrocapillary wave diffraction (ECWD). Over a relatively small temperature range (40-70 degrees C), the damping constant was more dependent on temperature than on the wave vector. Different surface layer properties were observed for two PDMS : one terminated with a methyl group (PDMS-CH3) and the one terminated with a hydroxy group PDMS-OH). PDMS-OH showed a larger reduction in surface tension compared to PDMS-CH3 because of a higher interfacial affinity with PEG, which indicated that its monolayer surface activity was weaker. A lower sur face elasticity of PDMS-OH than that of PDMS-CH3 supported that conclusion. The block copolymer showed no surface activity, since the short siloxane moiety did not repel the strong interaction between PEG and the ethylene oxide.