Ink-jet formulations that have been designed to phase separate upon deposition onto the substrate have been investigated using model fluids. A physical model of expected separation behavior for an aqueous solution of butanol suggested that the butanol-rich phase would form at the contact line as an annular ring. The surface tensions in air of the saturated fluids and the interfacial tension between them were combined with measured contact angles on the substrate to show that the butanol-rich phase would partially undercut the water-rich phase. Printing trials with a 6 %wt butanol solution, however, did not show phase separation and the observed internal Marangoni flows were consistent with the more rapid evaporation of butanol than water. The origin of this behavior is the highly non-ideal vapor pressure for butanol at saturation. A criterion for predicting whether or not a particular mixture would phase separate was developed based on the non-ideal component vapor pressures. Aqueous glycol ether solutions show the correct properties and a 15.1 %wt dipropylene glycol methyl ether acetate solution displayed phase separation. Separation occurred first at the contact line and the phase boundary moved radially inwards until water evaporation was complete.