This work reports the effects of the bioflavinoids genistein and daidzein on lipid bilayers as determined by volume measurements, X-ray scattering, and molecular dynamics simulations. The experimental and simulated total molecular volumes were found to be in outstanding agreement with each other before the addition of genistein and daidzein and also after their addition. Both bioflavinoids inserted into the hydrocarbon region of both DOPC and diphytanoylPC near the carbonyls of the lipids and both decreased the bilayer thicknesses. The long axes of both bioflavinoids were oriented nearly parallel to the plane of the bilayer with their carbonyl groups preferentially pointed toward the proximal surface. A difference is that daidzein had a solubility limit of similar to 0.14 mol fraction in DOPC (similar to 0.12 mol fraction in diphytanoyIPC), whereas genistein was soluble at least to 0.20 mol fraction in both lipid membranes. Measurements of bending modulus K-C and simulation results for area compressibility modulus K-A indicate that both bioflavinoids soften bilayers.