The kinetics of physical gelation and the structure of telechelic ionomers have been studied by 1 H and 13 C solid-state NMR, SAXS, and WAXD. The reaction of MgO with unsaturated polyesters containing carboxylic and hydroxyl end groups leads to about a 10 3 -10 4 -fold viscosity increase in the initial solution of polymer in styrene. The results of the present study suggest that the main mechanism, involved in the viscosity increase, is determined by strong electrostatic interactions between Mg 2+ ions and the charged carboxylato chain ends, resulting in the formation of immobilized ionic multiplets and their aggregates or clusters. Molecular diffusion is the limiting step for cluster formation. SAXS, WAXD, and NMR results suggest a lamella-like structure of the clusters, which are composed of crystalline carboxylato-Mg-aqua complexes in the core part surrounded by a glassy polymer layer with a thickness of a few nanometers. Polyester chains interconnect multiplets, resulting in the formation of a physical network. The estimated distance between neighboring lamella is comparable with the end-to-end distance of network chains. The increase in the content of network chains and immobilized domains causes a large increase in viscosity and storage modulus. The effect of temperature and of ionic and hydroxyl containing additives on the thickening is discussed.