The excitation energy transfer from monomers to dimers of N,N＇-dioctadecyl thiacyanine incorporated in Langmuir-Blodgett (LB) films of Cd-arachidate or dipalmitoyl phosphatidic acid was studied as a function of the dye concentration, ranging from 0.5 to 6 mol %, to obtain insight in the two-dimensional distribution of the dye. The details of the energy transfer were determined by the global analysis of picosecond time-resolved fluorescence decays recorded on a single mixed monolayer. Those studies were correlated with scanning confocal fluorescence microscopy measurements. From the spectral decomposition of the absorption spectra, the fraction of molecules present as dimers or aggregates amounts to 0.2 and is constant over the dye concentration range studied. The spatially unresolved data are supported by the confocal experiments which show that the dye distribution in the LB films studied is inhomogeneous and suggests the formation of two separate phases : strongly fluorescing regions of high dye concentration, containing a mixture of monomers and dimers participating in the energy transfer process, are surrounded by regions of low fluorescence intensity and of small dye concentration containing almost exclusively dimers. Although in the Cd-arachidate matrix the fraction of dimers not participating in the energy transfer process amounts to 0.135 +/- 0.015 over the concentration range studied, this amount can be considered negligible in the dipalmitoyl phosphatidic acid matrix.