The 3-hydroxyl groups in (+)-catechin and (-)-epicatechin 5 : 7 : 3' : 4' tetramethyl ether are of opposite configuration as reductive ring-opening and methylation gave enantiomorphous pentamethoxy-1 : 3-diarylpropan-2-01s. Epimerisation of catechins and epicatechins therefore results from inversion of the 2-aryl and not of the 3-hydroxyl gr0up. Prelog's atrolactic acid method has been used to determine the absolute configurations of the catechins and efiicatechins, the first of such determinations with flavan derivatives. Hydroxyl stretching frequencies of (+)-catechin and ( -)-efiicatechin tetramethyl ethers indicate strong intramolecular hydrogen bonding of 3(a)-hydroxyl groups and hence the existence of these compounds in the preferred 2(a)aryl : S(a)hydroxyl and 2(e)aryI : 3(a)hydroxyl conformations respectively.