Six different process aids - naphthenic oil, paraffinic oil, aromatic oil, pine tar, white factice and CI resin - in natural rubber were evaluated for their effect on physical properties of vulcanizates, with special reference to those relevant to engineering applicationa. Creep, stress relaxation, set, hysteries, heat build-up and resilience were studied. It was found that naphthenic oil showed the optimum properties, where as CI resin gave the least desirable set of properties. These two process aids were further evaluated to study the effect of their concentration on properties. It was found that the engineering properties were improved by the addition of naphthenic oil upto 10 phr. However, CI resin showed abnormal behaviour. A rating of process aids was made as far as general engineering properties were concerned.