The interaction between oxidized (ubiquinone-10) and reduced (ubiquinol-10) coenzyme Q(10) with dimyristoylphosphatidylcholine has been examined by differential scanning microcalorimetry, X-ray diffraction, infrared spectroscopy, and H-2 NMR. Microcalorimetry experiments showed that ubiquinol-10 perturbed considerably more the phase transition of the phospholipids than ubiquinone-10, both forms giving rise to a shoulder of the main transition peak at lower temperatures. Small angle X-ray diffraction showed an increase in d-spacing suggesting a thicker membrane in the presence of both ubiquinone-10 and ubiquinol-10, below the phase transition and a remarkable broadening of the peaks indicating a loss of the repetitive pattern of the lipid multilamellar vesicles. Infrared spectroscopy showed an increase in wavenumbers of the maximum of the CH2 stretching vibration at temperatures below the phase transition, in the presence of ubiquinol-10, indicating an increase in the proportion of gauche isomers in the gel phase, whereas this effect was smaller for ubiquinone-10. A very small effect was observed at temperatures above the phase transition. H-2 NMR spectroscopy of perdeuterated DMPC showed only modest changes in the spectra of the phospholipids occasioned by the presence of coenzyme Q(10). These small changes were reflected, in the presence of ubiquinol-10, by a decrease in resolution indicating that the interaction between coenzyme Q and phospholipids changed the motion of the lipids. The change was also visible in the first spectral moment (M-1), which is related with membrane order, which was slightly decreased at temperatures below the phase transition especially with ubiquinol-10. A slight decrease in M, values was also observed above the phase transition but only for ubiquinol-10. These results can be interpreted to indicate that most ubiquinone-10 molecules are localized in the center of the bilayer, but a considerable proportion of ubiquinol-10 molecules may span the bilayer interacting more extensively with the phospholipid acyl chains.