We present a wavelength prediction procedure on the basis of X-ray measurements and simulations for InP-based injectorless quantum cascade laser (QCL) devices. These lasers show excellent performance in the mid-infrared wavelength range, but are very sensitive to growth deviations, which cause strong wavelength shifts and are a serious obstacle for applications like gas sensing. However, by XRD-simulations of the active region, which consists of InAs, AlAs, GaInAs and AlInAs, the thicknesses and compositions can be extracted and are used as input values for bandstructure calculations, so that a prediction for the resonance energy of the laser transition can be obtained. With this technique a wavelength evaluation of injectorless QCLs with 3% accuracy could be accomplished, which is an essential improvement in applications like gas sensing. (C) 2010 Elsevier B.V. All rights reserved.