Measurements of the rates of mass transfer in binary cryogenic distillation of the nitrogen-oxygen and the argon-oxygen systems by a packed column with structured packings under total reflux conditions were made for a wide range of vapor flow rates, concentrations, and total pressures. HETPs in cryogenic distillation show irregular scattering with vapor Reynolds numbers, and are observed to vary with oxygen concentration, whereas vapor phase diffusion fluxes were well correlated. A new correlation was proposed for the vapor phase diffusion fluxes for binary cryogenic distillation of the nitrogen-oxygen and the argon-oxygen systems by a packed column with structured packings. Prediction of separation performance of a binary cryogenic packed column distillation by simulation techniques based on the proposed local flux correlation was made to show good agreement with observed data. Application of the binary correlation to ternary cryogenic distillation of air by a pilot scale air separation plant is also discussed.