Abstract Catastrophic phase inversion is induced by changing the phase ratio in a liquid–liquid dispersion and is widely used during the dispersion stage in the production of aqueous polyurethane ionomer (PUI) colloids. In the work reported here, water was added to polyurethane ionomer prepolymer (PUIp) until the water became the continuous phase. Three different dispersion regions have been discovered by changing the ionic group content. Stable emulsions containing small polymer drops were produced in Region I. Stable coarse emulsions containing a mixture of drop structures were produced in Region II, but only temporary dispersions could be produced in Region III. Conductivity measurements could not always be used to detect the phase inversion points effectively because the PUIp was swollen by water. Therefore, torque change measurements have been used in conjunction with the conductivity measurements to detect the phase inversion points for all three dispersion regions. Scanning electron microscopy (SEM) and optical microscopy were used to obtain images of these dispersions in the different regions. A catastrophic phase inversion map is used to represent the changes that occur in the PUIp–W dispersions. This map is plotted using the ionic group content as the ordinate and water content (at the phase inversion points) as the abscissa.