The dose comparator apparatus (Kemp, 1946) has been used to measure backscatter factor and percentage depth dose for various field sizes at 50 cm. and 40 cm. F.S.D., using radiation of approximately 1·0 mm. Cu H.V.L. The results are in fairly good agreement with values given by Quimby and Laurence (1940), but there are considerable deviations (up to 30 per cent.) from the figures given in the Mayneord and Lamerton Survey (1941) for large field areas, the latter giving lower backscatter factors and higher percentage depth dose values at large depths. These deviations are shown to be mainly due to the weight given in the Mayneord and Lamerton Survey to results obtained in wax phantoms.Various factors affecting measurements in a phantom have been investigated. These factors include the effects of phantom material, of using a large ionization chamber, or a chamber with a heavy metal stem, and of applicator construction. A comparison of results with the same applicator used on different X-ray sets is also given.It is concluded that measurements with the dose comparator apparatus are free from any significant errors.