The mechanism whereby olefins undergo isomerization in the presence of sodium chloropalladate(II) has been shown to involve the stepwise movement of hydrogen atoms from the one carbon atom to its nearest neighbour. Evidence leading to this conclusion is furnished by an examination of the products of isomerization both by vapour phase chromatography which gave the product distribution, and by N.M.R. which, in conjunction with selective labelling of the olefin using deuterium, enable a picture to be drawn of the pattern of hydrogen migration.