The conductivity of floating monolayers of a charge transfer complex between bisethylenedioxytetrathiafulvalene (BEDO-TTF) and decyltetracyanoquinodimethane (C(10)TCNQ) mixed with icosanoic acid (C-20) was examined at the air-water interface as a function of the mixing ratio, The conductivity of the film in the region in which the surface pressure increased rapidly showed different behaviour in a.c. and d.c. measurements. This was related to the domain structure of the monolayer. The relationship between the surface pressure and the conductivity is discussed in terms of the surface structure of the monolayer.