The synthesis of Hantzsch ester 1,4-dihydrocol-lidine-3, 5-carboxylate (Ib) in buffered solutions at a pH of 3-10 has been studied. The ester is obtained in good yield (1-53%) at pH 3.25 to 5.0 in 1.0N acetic acid medium. This is in contrast with the observation of Haley and Maitland (1. Chem. Soc., 1951 , 3170). The yellow impurity rioted by them has been found to contain a pyridine derivative of Ib and another basic yellow compound. t3 -Aminocrotonic ester and ethylidene ethyl acetoacetate are the only intermediates during the formation of Ib. The 1,5-diketone on treatment with ammonia gives a compound C H NO ,and not Ib (as reported earlier by Knoevenagel and Klages.Annalen, 1894, 281:105).