The nickel(II) catalysed isomerisation reactions of D-fructose derivatives modified at positions 5 and/or 6 were investigated. 5,6-Dimodified open-chain D-fructose derivatives as well as an open-chain derivative of D-xylulose reacted to give complex mixtures containing no major product. 5-Modified ketohexoses invariantly were degraded to the corresponding methyl pentonates upon loss of C-1. 6-Modified D-fructofuranose furnished the desired rearrangement into a branched-chain derivative of D-ribose. In marked contrast to previous belief, from these results it appears that 5-OH plays an important role in the productive co-ordination of the D-fructose derivatives to the nickel-ethylenediamine complexes under consideration.