We report the results of extensive thermal magnetoresistance measurements on single-crystal and polycrystalline specimens of potassium having residual resistance ratios (RRR) ranging from 1100 to 5300. Measurements were made between 2 and 9 K for magnetic fields up to 1.8 T. The observed thermal magnetoresistance cannot be understood on the basis of either semiclassical theories or from the electrical magnetoresistance and the Wiedemann-Franz law. We do, however, observe a number of interesting relationships between the thermal and electrical magnetoresistances, many of which are not immediately obvious when comparing direct experimental observations. The thermal magnetoresistance W(T, H) is given reasonably well by W(T, H)T = W(T, 0)T + AH + BH 2, where both A and B are temperature-dependent coefficients. Our results show that A = A 0 + A 1 T 3, while B(T) cannot be expressed as any simple power law. A0 is quite dependent upon the RRR, while a 1 is independent of the RRR. We find two very interesting relationships between corresponding coefficients in the electrical and thermal magneto-resistance: (i) the Wiedmann-Franz law relates A 0 to the Kohler slope of the electrical magnetoresistance and (ii) the temperature-dependent portions of the electrical and thermal Kohler slopes are both proportional to the electron-phonon scattering contribution to the corresponding zero-field resistance. The latter provides evidence that inelastic scattering is very important in determining the temperature-dependent linear magnetoresistances. Part, but by no means all, of the quadratic thermal resistance is accounted for by lattice thermal conduction. We have not been successful in generating another mechanism that gives a quadratic field dependence. After subtracting the lattice contribution, the Lorenz ratio is still strongly field dependent, decreasing with increasing field. Based on these observations and additional arguments, our general conclusion is that at least a portion of the anomalous electrical and thermal magneto-resistances is due to intrinsic causes and not inhomogeneities or other macroscopic defects.