The reactions of isoprene and butadiene with SH radicals have been investigated by density functional theory and ab initio molecular orbital theories. We report the thermodynamics and kinetics of four different pathways, involving addition of SH radicals to all double-bonded carbon atoms. Calculations have been performed on all stationary points using BHandHLYP functional, Moller–Plesset perturbation theory to second-order (MP2) and the composite CBS-QB3 method at the MP2 optimized geometries and frequencies. Pre-reactive complexes have been identified. The apparent activation energies are negative for SH addition at the terminal carbon atoms and are slightly smaller than those for OH addition at the same positions. The calculated overall rate coefficient for butadiene+SH reaction at 298K is in excellent agreement with the only available experimentally measured value. Activation energies and overall rate coefficients at different temperatures are predicted for the first time for butadiene+SH and isoprene+SH reactions. The reactions of butadiene and isoprene with SH radicals were found to be about four times faster than with OH radicals.