Nickel(II) catalysed isomerisation reactions of C-5 modified derivatives of D-glucose as well as L-idose were investigated. 5-Azido-5-deoxy-D-glucofuranose was successfully isomerised into the corresponding D-mannoepimer in good yields. Contrasting this result, in the case of the 5-modified L-idofuranose probed, no evidence for successful epimerisation at C-2 could be found. However, this sugar was quantitatively rearranged into the corresponding L-sorbopyranose. Upon extended reaction periods, the latter underwent degradation to give the coresponding 4-modified methyl L-xylonate.