A window arrangement on a pressure pipe, with a casing in the train or at the end of the pressure pipe, said casing featuring flanges on diametrically opposing sides having radially directed passages, whose axes are standing perpendicular to the longitudinal axis of the pressure pipe and are located in a measurement plane for an x-ray measurement device, an x-ray source being associated to the one passage on the outer side and a receiver sensitive to X-rays to the other passage, and with window plates that are transmissive for X-rays which are sealingly arranged in the associated passage and are fixed in the passage with the aid of a fastening member and which consist of a material which is resistant against high temperatures and process-due etchings by chemically aggressive substances.