The mutual mixing effect in the LB films of four kinds of merocyanine dyes have been investigated by optical absorption and fluorescence measurements. In single component films, DSe, DS and 6-methyl DS (6Me-DS) can form J-like aggregates but not for DO. In [6Me-DS]-[DO] system, a sharp J-band peak linearly shifts to the longer wavelength for the replacement of 6Me-DS by DO. In [DS]-[DO] system, [DS]:[DO] 1:1 film shows a both sharp J-band absorption and fluorescence peak. The behavior are understood by the formation of the ’solid solution’ for [6Me-DS]-[DO] and the ’intermolecular compound’ for [DS]-[DO]. Furthermore, the spectrum of [DO]-[DSe] system shows the presence of the ’phase separation’. These results would relate to an interesting problem, what we may call a ’2-dimensional phase diagram’ in J-aggregates.