Hydrogenation of isoprene (2-methyl-1,3-butadiene) on heterogeneous Pd catalysts affords the three methylbutene isomers in high selectivity and in the ratio: 2-methyl-1-butene:3-methyl-1-butene:2-methyl-2-butene ⋍ 1:1:2. Small variations from this ratio are accounted for by changes in the probabilities of adding the second H atom to different ends of the two distinguishable π-allylic intermediates, which are the half-hydrogenated states. These changes originate in the effects of the methyl substituents on electron distribution within the π-allylic radicals, the preferred position of addition depending on the charge carried by the H atom. Literature results for isoprene hydrogenation on various heterogeneous Pd catalysts and by various metal complexes in solution are examined to see whether the methylbutene isomer distributions are compatible with the π-allylic mechanism, and what kind of charge the second H atom carries. When a Pd C catalyst is selectively poisoned by the triphenyl compounds of the Group 15 elements, the charge on the second H atom appears to decrease with increasing atomic mass of the element. A variety of homogeneous complex catalysts also seem to operate through π-allylic intermediates, the second H atom apparently carrying some degree of positive charge. In both homogeneous and heterogeneous systems, mass-transport limitation may allow the methylbutenes to isomerize, and hence their concentrations will move towards their equilibrium values. This problem is signalled by the occurrence of a low total alkene selectivity. Providing this possible complication is borne in mind, obedience of the isomer distribution to the predictions of the π-allylic mechanism can be used as a diagnostic test for its operation, and can indicate qualitatively what charge the second H atom bears as it adds to the π-allylic radical.