The cyanomethylidenic sugars 1 -3 (α-D-erythroconfiguration) reacted with various phosphorus nucleophiles, gave branched-chain phosphorus-bearing sugar derivatives. The reactions were not discriminating towards the geometrical isomerism of the starting material. For reactions implying the formation of a phosphoryl group (P-OR →P=O) a hydroxyl group was necessary, either in the solvent or the C-5 position of the substrate. In the cases where the substrate was not hydroxylated at C-5, an exo attack leading to the D-xyloisomer took place when kinetic control was operating, whereas ″thermodynamic control led to the endo (D-ribo) isomer. A hydroxyl group at the C-5 position of the substrate directed the nucleophilic conjugate addition towards the exo face. When an asymmetric phosphorus atom was formed (cyclic compounds9and10), the epimers could be separated and their configuration established. The benzoylmethylidenic sugar12behaved somewhat differently because of the larger charge delocalization in the intermediate enolate than in its nitrogen analog. This enolate effected readily (presumably intramolecularly) the elimination reaction leading to phosphoryl formation. No hydroxyl group was necccessary. The relationship between the steric course of the reaction and its type of control which supposes that elimination occurs much slower than addition did not apply here.