Non-geometric string backgrounds were proposed to be related to a non-associative deformation of the space-time geometry. In this talk, using the flux formulation of double field theory (DFT), the structure of non-associative deformations is analyzed in detail. It is argued that on-shell there should not be any violation of associativity in the effective DFT action. I discuss two possible non-associative deformations of DFT featuring two different ways how on-shell associativity can still be kept.