There is a problem in the neutron flux calculation in regions with a strong spectral transition from epithermal toward thermal. Space-dependent group constants are developed for the thermal range to treat the highly nonseparable space- and energy-dependent flux distribution that characterizes the transition of fast neutron spectra into partially thermalized spectra. The weighting spectra are obtained from a parametric application of the heavy gas model for scattering with absorption cross sections that include the resonances near and below 1 eV. A space dependence is introduced into weighting spectra by relating the parametric solution of the zero-dimensional spectral equation to thermal and epithermal group fluxes obtained from a prior one-dimensional diffusion calculation. Subsequently, space-dependent thermal group constants are generated. The method is implemented in a standard multigroup diffusion code, executed iteratively. This procedure was applied to compact liquid-metal reactor designs having thermalizing reflector regions. The results indicate the effect of global parameters such as the size of the thermalizing reflector on the group constants, which are considerably different from the classical local group constants.