PURPOSE: To prepare chloroformic acid ester easily in high yield and without directly using phosgene, by contacting Cl2 with CO in the presence of phosphine oxides in a solvent, and reacting the solution thus obtained with an alcohol.

CONSTITUTION: Chloroformic acid ester is prepared by contacting Cl2 with CO in a solvent in the presence of a catalyst selected from a compound I (R is 1W18C alkyl, 5W10C cycloalkyl) such as triethyl phosphine oxide, a compound II such as tetramethyl diphosphine dioxide, a compound III (R1 is 4W6C alkylene) such as 1-methylphosphorane oxide, phosphine oxide of IV such as ethylenebis (diethylphoshine) dioxide, and phosphine chloride of V, VI such as trimethyl phosphine dichloride; followed by reacting an alcohol such as n-butanol, with the solution thus obtained.

USE: Intermediates of pharmaceuticals, agrochemicals, and dyestuffs