The calculation of the optical functions for Ga x Al 1Àx N (x 0X85 and 0.65) solid alloys is conducted using the ab initio norm-conserving pseudopotential (NCPP) and linear combination of atomic orbitals (LCAO) methods. Simultaneously, we have carried out experimental investigations of the reflection spectra and using Kramers-Kronig relations the imaginary part of the dielectric susceptibility is evaluated. Calculations of the solid alloys are performed using the superposition of the particular pure single crystalline compounds with an appropriate weighting factor. Calculations are performed as a function of energy cut-off as well as the degree of orthogonalization of the normconserving pseudo-wavefunction to the LCAO wavefunctions. The optical functions at zero frequency give an overestimated value compared with the experimental data. The best agreement with the experiment can be achieved by using the orthogonalization procedure between the pseudowave and LCAO wavefunctions. The latter ones help to understand the appearance of incommensurate modulations in the mentioned compounds. The possibility of achieving good agreement between experimental and theoretical data is shown.

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