The subband structure and the intersubband optical-absorption spectrum are calculated in n-channel inversion and accumulation layers on the Si (100) surface in magnetic fields tilted from the direction normal to the surface. An approximation scheme based on the local-density-functional theory is employed. Effects of magnetic fields on the exchange-correlation potential are completely neglected. Combined intersubband-cyclotron transitions between different Landau levels associated with the ground and excited subbands are allowed in addition to the main transition between the same Landau levels. When the amplitudes of the combined resonances are sufficiently small, they are not affected by the depolarization effect, while the main transition is fully influenced and its position is shifted from the corresponding subband energy separation. This is shown to be also true of accumulation layers, where the subband structure is very complicated because higher quasicontinuum subbands lie close to the ground and the first excited subband. The agreement between the theory and recent experiments of Beinvogl and Koch is satisfactory concerning relative positions of the main and combined transitions in accumulation layers. This indicates that our calculations of both the depolarization effect and the subband energy separations are essentially correct. There remain disagreements especially concerning amplitudes of the combined resonances. © 1979 The American Physical Society.