Many biologically significant molecules interact with cell membranes. It is therefore important to study the mechanisms of interaction, preferably at the molecular level. A key parameter characterizing these interactions is partial molar volume. This article shows how the partial molar volumes of the membrane lipid and solutes partitioning into a lipid bilayer membrane can be determined from the membrane refractive index, which in turn is obtained from measuring the velocity of light guided along a planar optical waveguide on which the membrane has been deposited. The technique allows the optical parameters for a single bilayer to be measured. By arranging the supported membrane such that it forms one wall of a cuvette, changes in these parameters due to the incorporation into the membrane of solutes introduced into the cuvette can be determined. For the lecithin-dibucaine system, three regions of behavior were found. At low mole fractions x of incorporated solute (up to x approximately 0.08), the dibucaine partial molar volume increases rather steeply, starting from a value only about half the estimated molecular volume, showing that the dibucaine is incorporated into preexisting voids in the bilayer membrane. In the second region (0.08 < x < 0.18), the dibucaine partial molar volume reaches a maximum, corresponding to its theoretical ''vacuum'' value, and then starts to decrease. Throughout these regions, the lipid partial molar volume changes by less than 1 %, but in the third region (x > 0.18), the lipid partial molar volume starts to increase, apparently as a result of extensive water penetration the bilayer due to the presence of the dibucaine, which is a prelude to the bilayer destruction and micellization observed at x > 0.3.