Summary It is important to specify the beam quality in a simple and nonambiguous way in order on one hand to make comparisons easier between treatments performed in various hospitals, or at different times in the same hospital and on the other hand to facilitate the choice of numerical values for factors like restricted masscollision stopping-power ratios and perturbation correction factors used in the conversion of ionization measurements into absorbed dose. We have adopted for high-energy photon beam specification a quality index ( I) defined by the ratio ( I 20/ I 10) of ionizations measured with a constant source-detector distance for a reference field size 10×10 cm 2. We have found that this quality index is independent of the source detector distance. On the other hand, the apparent linear attenuation coefficient measured on the exponential part of the tissue-maximum ratio curve can be calculated for any field size from the value of I for most high energy photon beams. In order to check the validity of the quality index for other linacs from other manufacturers, we have compared our results to published data related to various photon beams in a wide energy range: 2.5 to 45 MV.