This paper aims to shed light on the microstructure of tough, "double-network" (DN) hydrogels synthesized by free-radical polymerization of a monomer within a highly cross-linked polyelectrolyte hydrogel and to discuss the most efficient topological microstructure for toughness enhancement. Fourier transform infrared (FTIR) characterization of a hydrogel synthesized from the potassium salt of 3-sulfopropyl acrylate (SAPS) and 2-hydroxyethyl acrylate (HEA) demonstrated that polymer chains synthesized during the second polymerization step of a conventional DN hydrogel are grafted to the skeleton of the polyelectrolyte network. Uniaxial tensile tests performed on hydrogels synthesized from SAPS and acrylamide (AAm) indicate that linear and nonlinear polymerization of a second monomer within a network without grafting to the first network, i.e., forming a semi-interpenetrating or interpenetrating network, does not produce a tough hydrogel. Toughness enhancement of a covalent hydrogel was optimized by grafting high molecular weight polymer chains with a free end to a first, highly cross-linked polyelectrolyte network with residual unsaturation. The concentration of the grafted chains is a crucial factor in determining the mechanical behavior of the hydrogel.