In this work, the structure-property relationships of the thermal gelation of partially hydrolyzed polyacrylamide (PHPA) and polyethylenimine (PEI) mixtures were investigated under realistic conditions of temperature (80 degrees C) and salinity (total dissolved solids = 3.4 g/l) of the Algerian reservoir (Tin Fouye Tabankort) prior to a conformance control application. The reactants were characterized with regard to their hydrolysis degree or branching degree using C-13-nuclear magnetic resonance, and viscosity-average molecular weights (M over bar v) were estimated using the Mark-Houwink equation and intrinsic viscosities measurements. The polymers had molecular weights that varied from 5 to 10 x 10(6) g/mol for PHPAs with initial hydrolysis degrees between 6 and 20 mol%, while the molecular weights of the PEI were between 2 and 67 x 10(4) g/mol with a constant branching degree of 57-59. Consequently, the effect of steady shear on the gelation time was investigated followed by the effect of reactant concentrations, the polymer and cross-linker molecular weights, the polymer's hydrolysis degree, the temperature and the initial pH. All experiments were conducted in a semidilute concentration regime while maintaining practical initial gelant viscosities. As a result, the gelation time was found to decrease with reactant concentrations, molecular weights and temperature (E-a = 62 kJ/mol) and to increase with hydrolysis degree.