The　Langmuir　monolayer　of　carboxylic　methyl　ester　Gemini　surfactants　with　the　azobenzene　spacer,　referred　to　as　MCm(azo)MCm,　was　prepared　and　the　π-A　isotherms　were　measured.　The　result　revealed　an　orientational　picture　of　the　azobenzene　spacer　at　the　air/water　interface.　Before　irradiation,　the　planar　trans-azobenzene　group　adopted　an　approximate　configuration　parallel　to　the　interface　and　lay　on　the　air/water　interface.　After　UV-light　irradiation,　the　spacer　became　the　twisted　cis-one.　Because　of　the　location　of　the　polar　headgroups　at　the　air/water　interface,　the　two　phenyl　rings　were　also　forced　to　take　a　near-parallel　orientation　with　the　interface　to　reduce　the　free　energy.　As　a　result,　the　spacer　thin-layer　was　＂thickened＂,　which　caused　an　elevation　in　collapse　pressure.　©　2010　SIOC,　CAS,　Shanghai,　&　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.