The most widely applied method for the design of mixed vapour condensers is the approximate method of Silver II, Trans. Inst. Chem. Engng 25, 30-42 (1947)] as modified by Bell and Ghaly [2, AIChE Symp. Ser. 69, 72-79 (1972)]. However the older, and physically more realistic method of Colburn and Hougen [3, Ind. Engng Chem. 26, 1178-1182 (1934)] and Colburn and Drew [4, Trans. AIChemE 33, 197-215 (1937)] is often applied to the case of binary condensation. The relationship between these two methods is treated in a more complete and useful manner than in previous work. There has been an assumption that the two methods are equivalent at Lewis numbers near unity, but the approximate method may then be unsafe in design by up to 50%. The predictions are even less safe when the Lewis number is greater than unity, and cases are reported where the methods show discrepancies as large as 150% in gas film heal transfer coefficients. Discrepancies of similar magnitude are also found when the Lewis number is much less than unity, but the approximate method is then safe. The improved understanding that is reached leads to a new correction factor allowing the approximate method to be brought into good agreement with the Colburn approach in a self-consistent manner. The work is validated by comparison with the experimental data of Lehr [5, Doktor Dissertation Warme und Stoffubergang bei der Kondensation von Dampfen aus einem Gemisch mit einem Inertgas, Technische Universitat, Hannover (1972)].