Low molar mass polybutadiene was made crosslinkable under moisture by the addition of mercapto propyl triethoxy silane to double bonds. The reaction was carried out at 75 degrees C in the presence of AIBN as initiator. AIBN concentration and time showed that triethoxy silane groups were introduced quantitatively in the range of 1.05 to 7.9 per chain. A H-1 n.m.r. study showed clearly that only vinylic units reacted at low modification rates. Volumetric titration of the remaining thiol was used to follow the reaction. A new mechanism is proposed here to lead to a kinetic relation, similar to the one already used by several authors, but kinetically provable. Moreover, it has been pointed out that there is probably an evolution of kinetic constants with the progress of the reaction due to, among other reasons, a local steric hindrance at the level of vinyl sequences and/or a variation in chain conformation.