The non-linear load-deform ation behaviour of textile m embranes highly depends on the ratio of the applied m embrane forces in warp and weft direct ion (called load ratio hereafter). In practice, usually for each membrane structure the biaxial material behaviour is determined experimentally. The Japane se Standard MSAJ/M-02-1995 describes a standardized biaxial testing procedure. To achie ve input parameters for the structural design process, the commentary to this standard expl ains some methods how to evaluate one set of fictitious elastic constants based on the expe rimental results which, sim ultaneously, envelop different load ratios and do not reflect the non -linear material behaviour anymore. Different approaches of determining such simplified, fictit ious elastic constants have been investigated in the present contribution, with m ainly two co nclusions: firstly, to have one set of elastic constants by means of which all types of structures under all types of loading can be treated is a highly disputable objective a nd secondly, the values of the determined elastic constants react quite sensitively on the underlying determination option, which should be defined by the users themselves.