The thermal expansion coefficients (TECs) of B1 structured Ti1 − xAlxN and Cr1 − xAlxN thin films – investigated by synchrotron X-ray diffraction from room temperature to 600 °C – excellently agree with ab initio obtained temperature dependent calculations only if they were annealed at 600 °C. As-deposited thin films, with their built-in structural defects show a lower temperature dependence of their TECs and higher values. Furthermore, our data clearly show that the TECs of cubic Ti1 − xAlxN and Cr1 − xAlxN increase with increasing Al content, and that the TEC of wurtzite type B4 structured AlN is only about half of that of B1 AlN.