© 1994 SPIE. All rights reserved. Quantitative determination of individual microdomain sizes in Langmuir-Blodgett films of fatty acid binary mixtures has been accomplished using IR spectroscopy. Calculations based on the magnitude of the CH2 and CD2 scissoring band splitting showed that the protiated fatty acid domains consists of 80 to 100 chains for a lignoceric acid and deuterated stearic acid binary mixture. The domain size decreases as the chain length difference of the components decreases. The largest domains occur when the protiated fatty acid is the major component and the chain length difference is the greatest.