Using FTIR, dynamic light scattering, and electrical conductivity measurements, the effect of the addition of two different poly(vinylpyrrolidone) polymers, PVP K15 (MW = 10000 g/mol) and PVP K25 (MW = 24000 g/mol), on the structure and size of Aerosol OT/isooctane water-in-oil microemulsions has been examined. Results show that the addition of these polymers may induce the formation of different kinds of microemulsions. In the presence of PVP K15, the apparent hydrodynamic radius decreases with the increase of the polymer concentration. However, when PVP K25 is confined inside the Aerosol OT microemulsion, the apparent hydrodynamic radius decreases with the polymer content until a polymer concentration of 1% (w/w). Above this concentration, the radius increases as the concentration of PVP K25 increases. At a critical polymer concentration (>2%), the growth of the microemulsion is greater and a cloudy solution appears. FTIR spectra of these microemulsions show that structural differences between PVP K15-filled aggregates and the empty ones are very small, while the addition of PVP K25 may induce changes on the interface elasticity and on the distance between the surfactant sulfonate group and sodium counterions. From the electrical conductivity measurements no attractive interactions between polymer-filled droplets have been detected.