The two secondary standard dosemeters available at the National Laboratory of Ionizing Radiation Metrology (LNMRI) for calibration of parallel plate chambers used in the dosimetry of low-energy X-ray beams have calibration faetors for beam qualities up to 50 kV and half-value layer (HVL) of 2.257 mm AI. Since many superficial therapy X-ray tubes operate at 60 kV, for example the Chaoul tube which generates a beam of HVL equal to 3.3 mm AI, there is a demand of calibration of such chambers in beams generated at this potential. This work, aiming at the calibration of parallel plate chambers at 60 kV, evaluates the difference of the exposure rate, at the same point, for two lowenergy beam qualities (HVLs equal to 0.5 and 1.0 mm AI respectively), as determined by a parallel plate and a cylindrical standard chambers which have calibration factors for this energy range.