Lipid bilayer permeation coefficients for the neutral maleic acid molecule and the maleate monoanion have been determined by proton magnetic resonance techniques. Phosphatiydylcholine-cholesterol (2:1) unilamellar vesicles were prepared having an initial maleate anion concentration gradient stabilized by coupling to an impermeant potassium counterion. The coupling was released by addition of valinomycin, and the time evolution of external pH, internal pH, and maleate concentration followed using nuclear magnetic resonance areas and chemical shifts. Transport rate equations were numerically integrated to fit the date, yielding best fit permeation coefficients of 4 X 10(-9) and 4 X 10(-5) cm/s for maleate monoanion and maleic acid, respectively.