The neutron balance of a reactor system is probably the most important single quantity to be obtained from an analysis of static core physics. In a heterogeneous reactor configuration, an accurate knowledge of the different reaction rates must be obtained by first studying the unit cell. The results for the unit cell are used in the homogenization of the reactor lattice so that a multigroup, multiregion reactor program may be used to investigate the reactions rates of the reactor system. A study was made of the ability of the Sn form of the neutron Transport Equation to describe accurately the thermal neutron flux distribution in a unit cell. The uncertainties introduced into the problem by spectrum hardening in heterogeneous cells were minimized by confining most of the comparison of theory with experiment to natural uranium rods in diphenyl and D2O. A slightly enriched uranium slab in a water lattice was used for comparisons of results published in reference (1). Results are evaluated on the basis of the Sn method's ability adequately to calculate the spatial variation of the thermal flux distribution when compared with experiment. Excellent agreement was obtained for the Sn calculations.