Addition of low amounts of additives is necessary to guarantee dimensional stability of low-density flexible polyethylene foam after extrusion. It is expected that the additives form a layer on the foam cell walls. However, the physical phenomena responsible for the stability improvement are not well understood. Using low-frequency dielectric spectroscopy, we show the occurrence of interfacial polarization in these low-density polyethylene foams. We hypothesize and prove that a stabilized low-density, closed cell polyethylene foam can be regarded as a three-phase system with a polyethylene matrix forming the cell walls, a filler being the gas phase, and an intermediate additive layer. At elevated temperatures the additive layer will become conductive resulting in interfacial polarization, thereby excluding the entire filler volume from the electrical field. This is reflected in an increase of the dielectric constant of the foam from initially 1.0 to about 300 at the melting point of the additive. The frequency-independent position of the peaks suggests that the additive layer is crystalline.