Dioctanoylphosphatidylcholine (diC8-PC) forms micellar dispersions in water. At a certain low-concentration range the solution phase separates into two micellar phases below a certain transition temperature T0, and the coexistence curve shows an upper consolute temperature T(c) at a lecithin molar fraction x(c). The phase separation phenomenon can be understood in terms of an adaptation of a thermodynamic model due to Blankschtein, Thurston, and Benedek (BTB). In this paper we report the results of a study of diC8-PC micellar solutions in a solvent consisting of D2O/H2O mixtures. The diC8-PC/D2O/H2O system shows phase separation with significant variations of T(c) and of the critical lipid molar fraction x(c), depending on the relative amount of D2O. This indicates that an increasing fraction of D2O in the solvent primarily affects the system by modifying lipid-water interactions in such a way that the effective monomer-monomer interaction increases.