A liquid mixture, comprising a) at least one compound selected from polyols, polyol ethers wherein at least one hydroxyl group is unetherified, or polyalkylene ethers wherein at least one terminal hydroxyl group is unetherified, b) 1,3-bis(caprolactam-1-yl) butane, and c) diacetoxybutane mimics the solubility properties of a graft copolymer comprising a poly(alkylene glycol) backbone and a vinyl acetate/N-vinylcaprolactam copolymer grafted onto the backbone. In a method for evaluating the compatibility of a biologically active substance with the graft copolymer i) the biologically active sub-stance is brought into contact with the liquid mixture, and ii) the phase behavior of the test system and/or the solubility of the biologically active substance in the mixture is determined.