The visible absorption spectra of N,N′-distearylquinocarbocyanine iodide (SQCC) incorporated into egg yolk phosphatidylcholine liposome bilayers were examined, varying the SQCC/phosphatidylcholine molar ratio from 1:5900 to 1:62, in the temperature range from -15 to 30°C. Reversible thermal changes in the spectra were observed, particularly between two characteristic temperatures depending on the molar ratio of SQCC/ phosphatidylcholine and interpreted in terms of the dimer formation of SQCC in the liposome bilayers. The stability of the SQCC dimer in the lipid bilayers was found to depend on the physical state of the acyl chains in the lipids which form the bilayers; the dimer form is preferable in the solid phase in which the acyl chains of the lipids are in the all-trans configuration, on the other hand, the monomer is more stable than the dimer when the acyl chains are in the fluid state. The dimerization constant of SQCC in the liposome bilayer was determined as 3.2 (L/mol) in the fluid phase (at 25°C), and 1.30 × 103 in the solid phase (at -10°C). In the intermediate temperature range, coexistance of the solid phase and fluid phase in each liposome bilayer is suggested by the fact that an apparent dimerization constant depends on the molar ratio of SQCC/phosphatidylcholine as well as on temperature.