Inception of bubble nucleation in falling films of binary mixtures were determined experimentally. The liquid films were distributed over the external surface of a vertical evaporator tube with OD of 38 mm and heated length of 3 m. Local wall superheats and heat fluxes were measured at the point of bubble nucleation for mixtures of glycerol and water, at absolute pressures varying from 25 to 434 mmHg. Film Reynolds numbers ranged from 82 to 1,800. The experimental results showed increasing wall superheat for boiling inception with decreasing absolute pressure, decreasing flow rate, and increasing concentration of the less volatile component (glycerol).