Drying of natural rubber (NR) crumb (grade TSR10) requires high temperatures (100-120 °C). In order to determine the changes in bulk viscosity during the drying process, ageing kinetics at 120 °C were studied on model NR samples. During the process, changes in Mooney viscosity and weight-average molar mass (Mw) were monitored. Rubber from clones GT 1 and PR 107, with a bimodal inherent molar mass distribution (MMD0), was degraded in a two-phase process. During the first phase, Mooney viscosity and Mw increased, undoubtedly owing to a predominance of storage hardening over chain scissions [0 < t (min) <120]. During the second phase, chain scissions predominated (t > 120 min) and Mooney viscosity and Mw decreased. For rubber samples from clone PB 217, with a unimodal MMD0, no or reduced storage hardening was observed throughout the ageing process. These results showed that the key parameter involved in storage hardening seems to be the quantity of short polyisoprene chains and probably the nature of the chain ends. Copyright © 2003 Society of Chemical Industry