With the first use of ETFE foils as room-enclosing roof elements in the Mangrove Hall of the Burger Zoo, Arnhem, the Netherlands, ETFE foils are a fixed component in façade and roof systems of prestigious buildings and halls. Herein, ETFE foils can be applied in membrane structures independent of the building location’s local climate. For instance, at the Khan Shatyr Entertainment Center in Astana, Kazakhstan, temperatures can reach down to -50 °C in winter and up to +40 °C in summer. The design process focuses on uniaxially determined material characteristics, such as the ultimate tensile strength fu or the creep deformation at specific load levels. However, in both cases and independent of the climate, the biaxial material behaviour is relevant, due to membrane structures being stressed biaxially. Yet, uniaxial short-term and long-term tensile tests are much easier to produce than biaxial tests regarding costs, test record and evaluation. To compensate the complexity of biaxal tests, a correlation between uniaxial and biaxial material behaviour of ETFE foils is desirable. In the frame of this contribution, first results of investigations into the correlation between the uniaxial and biaxial long-term creep behaviour at room temperature are presented considering ETFE foils of three different material producers and two foil thicknesses. The developed correlation for describing the biaxial creep deformation at specific load levels is based on the creep deformation under uniaxial loading. With this new method, it is possible to determine the biaxial creep deformation for biaxial stress ratios σextrusion:σtransversal on test results achieved from uniaxial tests.