The effect of the alkyl chain length on the monolayer properties for some members of copolysiloxanes possessing an 4-alkoxyphenyl benzoate core in the side chain at the air/water interface was examined by surface pressure-area measurements. The negative temperature dependency of plateau pressures can be explained as an increase in mobility of main chains and side groups at the higher temperature favoring both removal of the (CH3)(2) SiO segment from the water surface and the closest packing of the side groups.