We consider the possible use of combination frequencies seen in high-amplitude δ Scuti stars for mode identification. For this purpose, we extend a previous theory to obtain expressions for the relative amplitudes and phases of combination frequencies for the case where radius and surface normal variations are important. Although a term is present which is sensitive to the pulsation mode, this term is always combined with another one which depends on the physics of the outer layers. Whereas the terms can be separated in DA and DB white dwarfs using suitable assumptions, these assumptions are not valid in pulsating main-sequence stars. We also discuss the strong correlation of relative phase of the combination frequencies with frequency. We show that this correlation is a result of the non-sinusoidal nature of the dominant pulsation mode.