AbstractLiquid SO2sealed into tubes made of a fluorocarbon copolymer permeates the walls of the tube at a temperature‐dependent but accurately reproducible rate. Sulfur dioxide dispensers made in this way are called permeation tubes and are useful for calibrating instruments that measure SO2concentrations in air. The National Bureau of Standards calibrates SO2permeation tubes and makes them available as Standard Reference Materials. The permeation rate in a batch of nominally identical tubes varies enough that each Standard Reference Material tube must be individually calibrated. Changes in the length or radial dimensions of the tubes are much too small to explain most of this variation. An excellent (negative) correlation is found between the measured permeation rate and the density of the polymer (or weight per unit length). Since both the measured density and the permeation rate for this semi‐crystalline polymer depend upon morphological factors, but in different ways, x‐ray diffraction measurements of the thickness and orientation of the lamellar crystals were made and a mathematical model was set up to identify the morphological factors which can cause variations in the permeation