Small-angle neutron-scattering (SANS) measurements have been made for a series of aerosol-OT (AOT)-stabilised water-in-oil microemulsions. The intensity pattern has been used to extract a value for the radius of the water core, rw, using D2O to provide the required contrast profile. In heptane the radii are found to follow an approximately linear relationship with respect to the [D2O]/[AOT] concentration ratio, R. At 20 °C, and R= 20, the structure of the water-droplet system is dependent on the hydrocarbon chain length of the oil medium. The experimental SANS patterns show increasing discrepancies with a fitted function based on monodisperse spheres as the length of the alkane chain is increased from n-heptane to n-dodecane. This effect is attributed to polydispersity and indicates that the droplet-size distribution within these microemulsion systems is much larger than had previously been thought.