'Volcanising done here' or some variant of this is a common sign displayed on roadside sheds. We all know that some treatment is done to rubber to improve its properties. But what exactly is the chemical process that is carried out? How does it alter the structure of rubber at the molecular level? How can these changes be made in a controlled manner? Let us put on the glasses of a chemist and find the answers. Natural rubber is obtained from latex, an emulsion which oozes out from the inner bark of many trees. l Coagulated rubber on separation and drying gives crude rubber. This is a polymer of isoprene (2-methyl-l,3-butadiene), with molecular weights in the range of 300,000. Synthetic rubber produced by polymerization of 1,3-butadiene, chloroprene, isobutene, etc., are also known today. Rubber, at this stage, is soft, sticky and thermoplastic. It has low tensile strength and low elasticity. It is easy to understand the molecular structural origin for these properties. The substance is a mixture of polymeric chains with varying lengths. Most importantly, there is no crosslinking at all. As a result, the material, although known for several centuries, did not find any significant application. The properties of rubber can be dramatically altered by cross linking the polymer chains. This process, carried out with sul-fur, is known as Vulcanization.2 Charles Goodyear discovered the process of vulcanization in 1839,accidentally. While carrying out an experiment, he spilt a mixture of rubber and sulfur with other ingredients on a hot stove. Lo and behold, rubber had transformed into a tough and firm material. The key chemical modification is that sulfide bridges are created between adjacent chains (Figure 1). The crosslinking makes rubber non-sticky and improves its tensile strength. The material is no longer thermoplastic. These attractive physical properties of vulcanized rubber have revolutionised its applications.