Rubbers are a family of polymers with unique viscoelastic properties, which allow the material to undergo large reversible deformations at condition that they are cross-linked. Even though sulphur vulcanization of rubber was discovered more than 150 years ago, the mechanism is still not completely understood and depends strongly on rubber typology and additives. The present work, focused on natural rubber, which is subject to the so-called reversion phenomenon resulting in a decrease of crosslink density at high cure temperatures. We propose a simple but effective mechanistic kinetic model in the framework of vulcanization with sulphur, which takes into account the reactions leading to reversion. The model is deduced from a schematic representation of the actual reactions occurring during sulphur curing at fixed vulcanization temperature. A simplified scheme is proposed that considers the typical experimental behaviour of a rubber compound during rheometer tests. From such schematic representation, a set of differential equations is written, allowing the determination of the final crosslinking density, provided that the solution is found by means of appropriated numerical tool. The final aim is to predict the evolution of vulcanization degree for isothermal cure conditions and estimate kinetic constants of the reactions representing curing. The model is tested on rheometer curves obtained for natural rubber at 5 different temperatures (from 130 to 170°C) on a MD rheometer (RPA2000).The kinetic constants found within the mathematical differential model are finally evaluated through numerical non-linear least squares data fitting experimental values. Reversion phenomenon at high temperatures is reproduced as well.