Abstract: A method and apparatus for detecting with the aid of X-ray radiation, caused to act upon a film or the like (5), heterogeneities in joints between mutually adjacent sections of insulation (4) on pipe assemblies comprising an inner steel pipe (3), insulation (4), and a tubular jacket (2). The novel features of the invention reside in bringing the radiation transmitted from an X-ray radiation source (1) of low-energy type, in at least two mutually sequential exposure stages to contact the steel pipe at at least one point thereon or along at least one tangential line (T.sub.1, T.sub.2) and to pass through the insulation (4) along the whole of that part of the radius of the jacket (2) which lies between the contact point line and the inner defining surface (8) of the jacket; and in that the axis (6) of the X-ray radiation source (1) lies in a plane which is substantially perpendicular to the longitudinal axis of the steel pipe.