Halophilic malate dehydrogenase is a negatively charged protein that crystallises well in a dilution process following a complex interplay with the three components of NaCl-MPD-H2O solvents (MPD: 2-methyl-2,4-pentanediol). The process was characterised by measuring the folding state of the protein, its concentration, its apparent solubility and second virial coefficients in various NaCl-MPD-H2O ratios representative of the phase diagram of the system. The protein crystallises by vapour diffusion between a drop containing protein in aqueous NaCl and MPD and a bath containing a given percentage of MPD in water. The starting drop is bi-phasic, in which the protein is concentrated in the salt-rich phase. This phase evolves towards a solution less concentrated in both NaCl and protein but more concentrated in MPD, before the system is driven to a single-phase region, where crystals are obtained. The protein stability is preserved during its crystallisation. We showed from second virial coefficient measurements that crystal formation is correlated with a slow evolution from repulsive to attractive protein-protein interactions. The end of the vapour diffusion process between the drop and the bath corresponds to a decrease of the attractive interaction, which we suggest favours crystal growth. The application of these results to crystallisation of other charged proteins and nucleic acids is discussed.