A deformation-controlled triaxial apparatus has been developed in order to investigate the mechanical behaviour of alpine snow. Cylindrical specimens (58 mm in diameter, 126 mm in height) can be axially deformed by applying tensile or compressive strain rates between 2.4 × 10−7 s−1 and 2.4 × 10−2 s−1. Compressed air is used to laterally load the snow specimens with pressures of up to 40 k Pa.The volumetric deformation is found by carefully encapsulating the snow samples in an airtight Mylar foil and measuring the expelled pore air during sample deformation. The multidimensional stress–strain behaviour can therefore be determined, and constitutive theories of snow advanced. The device is located in the cold rooms of the Weissfluhjoch research station of the Swiss Federal Institute for Snow and Avalanche Research. Some 180 tests have been performed over the last four winters. The density of the snow specimens varied between 190 and 435 kg m−3. Part I of this two-part paper discusses the development of the apparatus and the applied testing methodology. Qualitative results are provided. In Part II, a detailed analysis of the results is presented and the viscoelastic properties of snow quantified.