Natural rubber (NR), which is derived from latex of Hevea brasiliensis tree, contains about 3;-5;of non-rubber constituents. The presence of these non-rubber constituents, which are mostly proteinaceous materials, affect the sensitivity of rubber to water, thereby affecting properties such as elastic modulus and stress-relaxation. Studies using vulcanised unfilled NR showed that changes in relative humidity of the sample gave variations in elastic modulus and stress-relaxation of vulcanisates. The variability was due to the presence of certain types of amino-acid; their presence increases the elastic modulus and reduces the rate of relaxation of NR vulcanisates. These were due to the formation of ionic crosslinks; drying the rubber intensifies the ionic interactions and effectively increases the apparent crosslink density, thereby reducing the rate of relaxation.