We report on the double-crystal X-ray diffraction analysis of AlGaAs/GaAs quantum wire structures. We show that X-ray scattering is very sensitive to the modulation of structural parameters parallel to the crystal surface under certain diffraction conditions. The experimental diffraction patterns exhibit quantum wire satellite peaks which allow us to determine the quantum wire period and the quantum wire width. Furthermore, we found that due to the finite lateral width of the quantum wires a partial asymmetric strain relaxation of the unit cell occurs resulting in an orthorhombic lattice deformation.