Catastrophic-phase inversion has been studied in surfactant-oil-water (S-O-W) systems. Polyisobutene was dissolved in cyclohexane in order to change the viscosity of the oil phase. The effect of oil-phase viscosity and stirrer speed on drop sizes before inversion, at the inversion point, and after inversion has been examined. It is shown that the formation of drops within drops (O/W/O) can give rise to a close packed system at the inversion point. Most attention has been given to the inversion W/O to O/W. There is evidence that the mechanism of drop formation changes as the oil viscosity increases; a change in Reynold’s number affects the drop type. The effective volume fraction of the dispersed phase is increased by the formation of O/W/O drops; this did not occur at low viscosity for the class of surfactant used here. The volume fraction of water required for inversion decreased as the oil viscosity increased. Before inversion, when the viscosity of the oil is 0.025 N s m-2, drop diameter, D(owo), is given by D(owo) = 0.08 exp (5f(w)/2) N-0.8 indicating that drop coalescence controls drop size. As the oil-phase viscosity increases, the relationship D(owo) is-proportional-to mu(c)-0.35N-0.75 is observed. After inversion, D(ow) is-proportional-to N-0.8 and D(ow) is related to D(owo) but D(owo) decreases with increase of oil viscosity, whereas D(ow) increases with oil viscosity. Analysis predicts a relationship between D(owo) and D(ow), a minimum value for D(owo) and a correlation between D(owo) and the volume fraction of the dispersed phase at inversion. When the oil-phase viscosity is high, the inversion O/W to W/O does not have the same mechanism as the inversion W/O to O/W.