Self-consistent calculations of the Rashba splitting both in inversion and accumulation InAs layers were carried out using a method based on reducing 6×6 and 8×8 Kane matrix equations to a Schrödinger-type equation. Disregarding the Γ7-band contribution yielded a splitting magnitude overestimated by 50%. The essentially nonlinear dependence of splitting on the two-dimensional (2D) wavevector k restricts the applicability of the Rashba parameter α (coefficient at the k-linear term in the spectrum), including its value at the Fermi level, because of a strong dependence of the latter on the approximations applied to the two-dimensional spectrum. The relative differences Δn/n of the spin-split subband populations, calculated for the inversion layer, were found to be 2-3 times lower than those measured by Matsuyama et al. (see text). The experimental study of accumulation InAs layers showed that the corresponding value Δn/n does not exceed the calculated one, ∼0.02. The approach employed to describe the 2D spectrum (including spin-orbit splitting) was also shown to be adequate when applied to the case of quasi-classical quantization in a classically self-consistent surface potential.