Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1−xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1−xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3×1018 cm−3 and 2.8×1019 cm−3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.