X-Ray and 1H N.M.R. studies on pyranoid rings 1,2- cis -fused to dioxolane rings in acetylated D -gluco- and D --galactopyranose derivatives demonstrate that the configuration of the dioxolane ring influences the conformation of the pyranoid ring in the D -gluco but not in the D -galactopyranose series. The crystal structure of 3,4,6-tri- O -acetyl-1,2- O -(R)--(l-cyano-ethylidene)-α- D -glucopyranose ( 1 ) and 3,4,6-tri- O -acetyl-1,2- O -( R )-(1-cyano-ethylidene)-α- D -galactopyranose ( 2 )have been determined by X-ray analysis. Lattice parameters for 1 are a=20.6021 (11), b=8.0438 (2), c=5.5541 (1) Å and β= 95.588 (3)° for a cell with P21 symmetry. These parameters for 2 are a=20.3361 (7), b=10.0907 (2), c=18.9115 (5) Å, β =112.399 (2)°, C2, with two crystallographycally independent molecules. The conformation of the pyranoid ring in both compounds can be described as flattened 4C 1 and that of the dioxolane ring as distorted E 1. The importance of the torsion angles for describing problems of configuration is remarked and the use of relative configurational angles is stressed. The 1H N.M.R. spectra of 1 and 2 and 3,4,6-tri- O -acetyl-1,2-O-(S)- and (R)-ethylidene-α- D -glucopyranose ( 5 and 7 ), 3,4,6-tri-O-acetyl--1,2-O-(S)- and ( R )-ethylidene-α- D -galactopyranose ( 6 and 8 ), and 3,4,6-tri- O -acetyl-1,2- O -( S )-and ( R )-benzylidene-α- D -glucopyranose ( 9 and 10 ) have been analyzed by using iterative computer methods and N.O.E. measurements. The results indicate that the major solution conformation of the pyranoid ring of the derivatives in the D -gluco series 1 , 5 and 9 may be described as flattened 4C 1 and that of 7 and 10 as 2 S 5. The major solution conformation of the pyranoid ring in all compounds in the D -galacto series ( 2 , 4 , 6 , 8 ) may be described as flattened 4 C 1.