The microstructure in a ternary microemulsion, composed of pentaethylene glycol dodecyl ether (C(12)E(5)), water, and decane, was investigated by small angle neutron scattering along a dilution line defined by a constant surfactant-to-oil ratio, phi(s)/phi(o) = 0.815, where phi(s) and phi(o) are the surfactant and oil volume fractions, respectively. In the experiments three different contrasts were applied and the concentration was varied in the range 0.02 not less than or equal to phi less than or equal to 0.22, where phi = phi(s) + phi(o). At lower temperatures the microemulsion phase coexists with excess oil (emulsification failure). Along the emulsification failure phase boundary the results are consistent with a structure of spherical oil droplets, the size of which do not change with concentration. A simultaneous fit to the three different contrasts gives a droplet hydrocarbon radius of [r(hc)] = 75 Angstrom and a relative polydispersity sigma/r(hc) = 16%. When increasing the temperature, data are consistent with an increase in micellar size and a deviation from spherical shape.