The internal structure of water-in-oil microemulsion droplets has been studied by small-angle neutron scattering (SANS). Dichained surfactants with different tail structures were investigated : synthetic phosphatidylcholines, dialkyldimethylammonium bromides, and Aerosol-OT. The oil was cyclohexane, but for AOT n-heptane was used. The data from core-shell-drop neutron contrast series were globally analyzed to investigate the possibilities that water and/or oil penetrates the curved surfactant film. Three different models for the interfacial structure were tested : sharp-step as well as linear or exponential changes in neutron-scattering length density. Penetration of water was not detected. For phosphatidylcholine and n-octadecyl-n-dodecyldimethylammonium bromide layers, the SANS data were most consistent with a volume fraction of cyclohexane phi(C6D12) between about 0.05 and 0.20. However, for DDAB and AOT there was no clear evidence for any significant oil mixing. These results indicate that the extent of alkane penetration into such negatively curved monolayers depends somewhat on the surfactant alkyl chain structure.