X-ray reflectometry (XRR) is a well established technique to evaluate quantitatively electron density, thickness and roughness of thin layers. In this paper, results of the first world-wide XRR round-robin experiment, involving 20 laboratories, are presented and discussed. The round-robin experiment was performed within the framework of the VAMAS Project `X-ray reflectivity measurements for evaluation of thin films and multilayers', the aim of which is to produce a `good practice' manual for XRR. The reproducibility of measurements obtained using different equipment has been investigated. The influence of the fitting of the experimental data was shown to be non-negligible compared with the experimental factors. The dynamic intensity range proves to be an important parameter for obtaining a good quality measurement. A simpler test sample which does not develop a surface oxide layer over time is now the subject …