The dependence of the activation energy on the subband splitting and on the perpendicular magnetic field in silicon inversion layers at low carrier densities has been studied. The subband splitting was first varied by changing the substrate bias. After exhibiting an initial drop at the smaller values of the subband splitting, the activation energy saturates and even shows a slight increase with increasing reverse substrate bias at the lowest carrier densities. While this increase is attributed to the increased surface roughness scattering at higher values of the surface field, the initial behaviour can be explained by the existence of the very long band tails associated with the primed subbands. Our magnetic field measurements were conducted in a weak-field limit. We have found an increase of the activation energy with the magnetic field, which proceeds in a nonlinear fashion and can not be explained merely by the change of the subband splitting in a magnetic field. © 1988.