Lattice mismatch effects are obscured by the standard envelope wave‐function approach to electronic states because the lattice potential does not appear. A theoretical analysis of the first‐order effect of lattice mismatch is presented using the deformable ion approximation. It is found that a potential barrier of height &Dgr;, normally associated with a band offset, acquires an additional energy (E) dependent term becoming &Dgr;+&eegr;(&Dgr;−E) where &eegr; is the fractional lattice mismatch in the growth direction. For strained layers, &Dgr; includes the effect of deformation in the strained layer. Quantitatively, the lattice mismatch effect can be comparable to or larger than the deformation potential effect on the band offset. A scattering approach is employed.