For the heavy-gas model, the stationary space-dependent neutron spectrum in one- and two-dimensional heterogeneous thermal reactors is determined in the diffusion approximation. The fuel elements, which are not necessarily identical, and absorbing slabs or rods are arranged arbitrarily. However, absorption in all of them is assumed to follow a l/v law. The neutron flux is represented as a linear combination of the lowest eigenfunction of the Laplace operator for the geometry considered and a finite set of Green's functions for the stationary-wave equation for various, usually imaginary, wave numbers. The energy-dependent coefficients are determined by the author's method, developed in an earlier paper. The lowest eigenfunctions of the Laplace operator and Green's functions for the stationary-wave equation are given for some geometries of practical interest. Solutions found earlier for simple geometries may now be regarded as special representations of these Green's functions. But in these cases, too, other representations can be found which are to be preferred for numerical reasons.