Single-phase AOT/H2O/n-decane inverse microemulsions have been studied by using capillary viscometry and small-angle neutron scattering (SANS). This ternary mixture is treated as a polydisperse colloidal suspension, where the established overlap potential for the droplet interaction is approximated by a square-well interaction. Viscosity measurements on both dilute and concentrated microemulsions show an anomalous maximum with increased swelling of the droplets. Predictions for the dilute viscosity from the effective interaction parameters extracted from the microemulsion structures measured by SANS also show a maximum with swelling, but of a smaller magnitude. The viscometry and the SANS measurements are combined to extract the interparticle potential. Comparison shows, however, that two different sets of interaction parameters are extracted, a consequence of the approximate nature of the square-well colloidal model. A molecular mechanism is proposed to explain the viscosity anomaly, whereby disorder at the surfactant/water interface leads to increased overlap and, hence, stronger effective interactions. The results are discussed within the context of earlier work on the percolation transition in these inverse microemulsion systems.