The bending behaviour of inflated beams has been studied for several decades. Several models have been developed to predict the load deflection behaviour of the beams. The various models treat the problem differently, particularly in the way the wrinkling moment and collapse moment are defined. This paper relates the wrinkling threshold to a stress criterion rather than a strain criterion. An important issue with respect to the collapse load is whether to regard the material as a true membrane or as a very thin shell. It will be shown that the three thin films used in this research PC, PPS, and PEI should be regarded as thin shells. A new model that predicts the collapse moment for these kind of materials has been developed, which incorporates orthotropic material properties. These material properties were obtained via biaxial testing by means of axial loading of a pressurised tube. Experiments were conducted to obtain the load deflection curves at five different pressure levels. The results were shown to correlate reasonably well, with the proposed theory. (C) 2004 Elsevier Ltd. All rights reserved.