The crystallisation of the polymorphic salt, ethylene diammonium dinitrobenzoate, is reported. In particular it is noted that in this monotropic system it is possible at certain temperature, pH and supersaturation conditions, for the stable form to crystallise directly rather than via the metastable triclinic polymorph. The underlying reason for this is sought via studies of transformation kinetics and speciation of the salt components. The ideas developed are extended to demonstrate, for the first time, the selection of additives for the control of selective polymorph crystallisation of a molecular salt. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.