Abstract NMR observation revealed that bidirectional conversion occurred between 3- monochloropropane-1, 2-diol (3-MCPD) and glycidol in the course of the analytical procedure of DFG standard method C-III 18 (09), option A; 3-MCPD was partly converted to glycidol at the transesterification step, and glycidol was converted partly to 3-MCPD at the derivatization step conducted at 80° C under acidic condition in the presence of NaCl. ...