The self-diffusion coefficients of all components of the quaternary microemulsion system water-cyclohexane-octyl monoglucoside (beta-C(8)G(1))-geraniol (trans-3,7-dimethyl-2,6-octadien-1-ol) have been measured along a well-defined linear path in the composition space, using the NMR pulsed-gradient spin-echo technique. The results have been used to derive information about the microstructure of the microemulsion and to calculate the composition (proportion of alcohol to surfactant molecules) of the mixed interfacial film separating the microscopic oil and water domains in the microemulsion. For the present system a phase inversion (2) under bar-3-(2) over bar occurs at a 1:1 oil-to-water volume ratio and constant overall mass fraction of surfactant plus alcohol (gamma = 0.05) by increasing delta, the relative mass fraction of alcohol in the amphiphile mixture (surfactant plus alcohol). For delta less than or equal to 0.25 the relative self-diffusion coefficient D-rel of water is about 2 orders of magnitude greater than D-rel of oil, indicating an oil-in-water (o/w) droplet microemulsion; for delta greater than or equal to 0.50 an opposite behavior of D-rel for oil and water is found, indicating a water-in-oil (w/o) droplet microemulsion. For delta values between 0.30 and 0.45, D-rel of water and oil exhibit a pronounced opposite dependence on delta, with a crossover near the middle of the three-phase body (delta approximate to 0.40), where D-rel is about 0.47 for both oil and water. This behavior implies that the transition from an o/w- to a w/o-droplet microemulsion proceeds via a bicontinuous microstructure with oil and water domains separated by a mixed interfacial amphiphile film of low mean curvature. The composition of this film has been calculated as a function of delta by two mutually independent methods for the droplet regime and the bicontinuous regime of the microemulsion, respectively. With increasing delta the ratio of geraniol to beta-C(8)G(1) molecules in the mixed film increases from about 1:5 at delta = 0.15 (o/w domain) up to 1:1 at delta = 0.60 (upper limit of w/o domain), amounting to about 1:2 for the state of the balanced microemulsion. These results demonstrate in a quantitative manner how the composition of the mixed interfacial film affects the structure of the oil and water domains in a quaternary microemulsion system.