There is a serious lack of instrumental studies of polymeric membrane ion sensors that would give reliable information on the chemistry behind the sensor response. These types of studies are quite challenging due to the low content of species of interest in the membrane material and due to specific features of plasticized polymeric sensor membranes. Mössbauer spectroscopy is a powerful tool for studying local chemical environment of the probe atom. In this research we explored the forms of iron existence in different PVC-plasticized membranes containing Fe-binding ligand and cation-exchanger. In order to be able to observe Mössbauer effect we employed ⁵⁷Fe-enriched sample solutions to soak the membranes and performed the measurements at 80 K to increase the recoil-free factor. It was found that the chemical forms of iron are different in membranes containing the ion-exchanger and those containing both ion-exchanger and the ligand. The relative content of iron in various forms was estimated. It was also observed that iron ions enter into the «dummy» membranes with no ligand and ion-exchanger, however in very small amounts.