Abstract: The pure (3E)-3-(methoxymethyl) alk-3-en-2-ones and (3E)-3-(ethoxymethyl) alk-3- en-2-ones are formed in the acid induced reaction of 4-hydroxy-3-methylenealkan-2-ones with methanol and ethanol, respectively. Key words: Baylis− Hillman reaction, stereoselectivity,(3E)-3-(alkoxymethyl) alk-3-en-2-ones, trisubstituted alkenes, alcoholysis