Dosimetry of low energy x-rays is performed more and more often with the use of thin window parallel-plate ionization chambers because of their favourable performance characteristics compared with cylindrical chambers. The thin window is particularly attrao live for measuring beam parameters at the skin (phantom) surface but it also introducer dosimetric problems by allowing the secondary photoelectrons produced in the beam collimating system to enter the chamber. This may result in erroneous dosimetric data. This paper deals with some o f the problems caused by the secondary electrons and their effect on absorbed dose and depth dose measurements. To minimize these effects, it is suggested to increase the window thickness of the ionization chamber employed thus making the chamber only x-ray sensitive.