Dose measurements based on methylene blue (MB) bleaching, widely used for ultraviolet light, can also be applied to X-rays including very high flux levels. This method has been tested by using both MB bleaching and Fricke dosimetry for a conventional monochromatic X-ray source and then for 'white-beam' synchrotron radiation. The results show that MB bleaching dosimetry can easily measure X-ray doses up to at least 10(5) Gy s(-1), as long as the MB concentration is sufficiently high. This condition can be verified from the deviations from linearity of the bleaching versus exposure time.