Abstract: [ 4 ? ? Q air ? ? ? K ? ( P P 0 ) ? 0.35 ] ? 1 / 2 ( I ) In a process for the separation of air into at least one constituent by cryogenic distillation of air in a system of columns (1, 3) comprising at least one column (1, 3) for producing at least gaseous oxygen and/or nitrogen as product, if the operating pressure of the largest-diameter column is P>2 bar abs, then the diameter (in metres) of this column is less than (I), where : Qair is the sum of the flow rates in Sm3/h of the air feeding the column system and K is the capacity per unit area of the column and P0 is equal to 2 bar abs.