Optical absorption and fluorescence, as well as electron spin resonance measurements were performed on Langmuir-Blodgett films of binary mixtures of three kinds of merocyanine dyes, i.e. DO, DS and DSe, where DS and DSe form J-like aggregates (but DO does not) in single-component films. The observed optical absorption spectra of mixed films were markedly dependent on the combination of dyes. [DS](1-x)[DSe](x) LB films show a sharp red-shifted J-like band peak over the entire concentration range. In contrast, the spectrum of the [DSe](1-x)[DO](x) system consists essentially of [DO]’s monomer peak and [DSe]’s J-like band peak, suggesting the presence of phase separation. In the [DS](1-x)[DO](x) system, a J-like band peak is maintained up to a composition that contains 70% DO. In particular, [DS](0.5)[DO](0.5) film shows a sharp J-like absorption and a sharp J-band fluorescence peak, and a small Stokes shift is observed. These results suggest that most of the DO molecules associate to form J-like aggregates in the [DS](1-x)[DO](x) system.