This paper is focused on the characterization of polyelectrolyte-modified inverse microemulsions and their use as templates for the synthesis of magnetite nanoparticles. It is shown that the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) of low molar mass can be incorporated into the individual inverse microemulsion droplets (L2 phase) consisting of heptanol, water, and an amphoteric surfactant with a sulfobetaine head group. Up to a polymer concentration of 20% by weight in the aqueous phase and for different molecular weights of the polymer, an isotropic phase still exists. At a PDADMAC concentration of 10% the area of the isotropic L2 phase is shifted in direction to the water corner. In the percolated area of the L2 phase, i.e., at higher water content, a temperature-dependent change in the conductivity can by observed, and bulk water can be detected by means of differential scanning calorimetry measurements. The unusual temperature-dependent behavior of the polymer-modified system, i.e., the conductivity decrease with increasing temperature, can be explained by temperature-sensitive polyelectrolyte-surfactant interactions, influencing the droplet-droplet interactions. These PDADMAC-modified microemulsions can be successfully used as a template for the formation of ultrafine magnetite particles, in contrast to the nonmodified microemulsion, where the process is misdirected due to the "disturbing" effect of the surfactants. However, in the presence of PDADMAC the surfactant head groups were masked, and therefore magnetite can be synthesized. During the process of magnetite formation the PDADMAC controls the particle growing and stabilizes spherical magnetite particles with a diameter of 17 nm, which can be redispersed without a change in size.