Natural rubber is usually considered to have good processing properties. Although it is tough and nervy at room temperatures well below 100 C, it breaks down on mastication to a usable plasticity. The viscosity-stabilized grades of natural rubber do not generally require premastication before the incorporation of fillers and other compounding ingredients. A short mastication time is common practice with the non-stabilized grades. Although the processability of natural rubber cannot be predicted by any single parameter, viscosity still remains the most widely used measure of processing quality. During mixing, good control of compound viscosity within fairly narrow limits is essential to ensure smooth operation during subsequent processing such as extrusion and injection moulding. Thus, at intermediate levels, viscosity is a useful guide to the processing behaviour of masticated rubber. Like other elastomers the physical properties of natural rubber vulcanizates are dependent on several variables such as compound viscosity, type and amound of fillers, degree of dispersion and type of crosslinking. The development of a commercially viable, lighter coloured, non-toxic grade of (proprietary) depolymerized rubber by the Rubber Research Institute of Sri Lanka lead to research and development work on potential industrial applications of this material as a viscosity modifier suitable for a range raw rubber blend formulations used mainly in the tyre industry. The results are encouraging and show that incorporation of depolymerized rubber into the formulations to replace processings oils can significantly reduce the viscosity of the raw rubber blends whilst the vulcanizates show considerable improvement in abrasion reistance. Further, the use of depolymerized rubber as a speciality rubber in raw rubber blends has been shown to improve certain other properties due to its vulcanizable characteristics. The effect of the proportion of depolymerized rubber on vulcanization characteristics is also discussed. It has also been shown that the use of depolymerized rubber enhances the processing performance on extrution, as measured by die swell, and lowers the power requirement without adversely affecting the properties of the compound. The results obtained with the vulcanized mixes demonstrate that the behaviour of depolymerized rubber is comparable to that of synthetic liquid polyisoprenes and is an improvement over ordinary processing oils. Clearly, a much larger world market for depolymerized rubber is potentially available if depolymerized rubber can improve on the above processing aids with respect to quality and price.