Abstract A systematic investigation has been made of the factors likely to promote porosity in vulcanized rubber. Of the methods developed for assessing the degree of porosity, measurement of the percentage decrease in specific gravity is shown to be the most satisfactory. The permeability of the sheet to moisture, the acetone extraction of aged samples, and visual examination also give some indication of the degree of porosity. Severe over-mastication, particularly on cold rolls, plasticizes rubber and increases the entrapment of gases during mixing. If the pressure applied during vulcanization is low, the volume decrease, which occurs as the rubber vulcanizes, causes further lowering of pressure; some of the entrapped gases are liberated, and as the rubber is partially set up free escape is prevented and pores are developed. The effect of over-mastication can also be produced by heat or oxidation treatment of the raw rubber. Mechanical treatment of the mix, such as sheeting from the mill at friction speed, calendering to a thick gage, or extrusion of thick articles may increase the tendency for porosity under the appropriate conditions. Softeners added to the mix produce the same effect as over-mastication in increasing porosity, whereas stiffening fillers oppose its production. When no mechanical pressure is applied during vulcanization, porosity is likely to develop, but a slight pressure may cause even worse porosity through preventing the free escape of gases during the soft stage before vulcanization commences. The degree of porosity depends on the conditions prevailing during the initial stages of vulcanization and is, in fact, a useful indication of setting up when scorching tests are being carried out. It is not influenced by the action of heat, in the later stages of vulcanization on the rubber-sulfur or oxygenated rubber complexes. Moisture, derived from the ingredients of the mix, the processing or the methods of vulcanization, increases the porosity under appropriate conditions. The use of ultra-accelerators or high-curing temperatures, tends to increase porosity of mixes liable to this defect, as the escaping gases are more completely entrapped by a rapid setting up of the rubber. Some applications of the results to methods of overcoming porosity under works conditions are given.