The light induced switching of self-assembled monolayers containing azobenzenes is observed in situ by the Kelvin probe method. The switching process is optimized by preparing monolayers from mixed solutions of azobenzene thiols and n-alkylene thiols. In a film the efficiency of the E/Z-isomerization depends on both the spacer length between azobenzene group and gold surface as well as the free volume in the film, tuned by the n-alkylene thiol fraction in the mixture. An optimum was found in a mixed self-assembling film containing 33% 4-fluoromethyl-3'-(10-carboxy-decyloxy) azobenzene in a dodecane thiol matrix. The structure of the films were characterized by FTIR and AFM.