This study confirms that 2,3,4-tri-O-acetyl-1,6-anhydro-beta-D-mannopyranose is an artifact produced during carbohydrate analysis. A new synthesis of 2,3,5-tri-O-acetyl-1,6-anhydro-beta-D-mannofuranose is also described, and a novel dimer, 1,6':6,1'-dianhydro-2,3:2',3'-di-O-isopropylidene-5,5'-di-O-(1-methox yethyl)-di - alpha-D-mannofuranose, has been isolated. The structure of the dimer is confirmed by X-ray analysis of a derivative, 1,6':6,1'-dianhydro-2,3:2',3'-di-O-isopropylidene-di-alpha-D-mannofur anose.