AbstractEquations are derived for predicting the velocity above which a jet forms when one Newtonian liquid is injected into a second stationary immiscible liquid. Comparison of the theory with experimental data obtained for fifteen liquid‐liquid systems and five nozzle diameters shows a mean error of 6.6%. The equation also predicts the jetting velocity for injection of liquids into gases and the critical velocity at which bubble formation changes from a constant volume to a constant frequency mechanism for gas injection into liquid