Syntheses of 4-hydroxy-2-methyltamoxifen (1-{4-[2-(dimethylamino)ethoxy]phenyl}-1-(4-hydroxy-2methylphenyl)-2-phenylbut-1-ene) by dehydration of diastereoisomeric triphenylbutan-1-ol precursors were compared. Crystal structures of these diastereoisomers have now both been determined. Acid treatment of either diastereoisomer gave predominantly the E olefin (8:1 trans–cis), consistent with a mechanism proceedings by way of a common carbenium ion intermediate whereby the 2-methyl group destabilises the transition state that would lead to Z-olefin. A synthesis of 4-hydroxy-2,3,5-trimethyltamoxifen proceeded similarly. The (E)-4-hydroxy-2-methyltamoxifen was more resistant to isomerisation into a Z–E mixture than was 4-hydroxytamoxifen but a 3:1 trans–cis equilibirum mixture could be obtained. The 4-hydroxy-2,3,5-trimethyltamoxifen gave the same ratio of isomers at equilibrium as did the 4-hydroxy-2-methyl compound but it isomerised at a faster rate. It was shown from the ¹H n.m.r. spectrum of the carbenium ion derived by trifluoromethanesulphonic acid treatment of the model compound 1-(4-methoxy-2-methylphenyl)-1-(4-methoxyphenyl)-2-phenylbut-1-ene that, in these systems, the aryl ring that is best able to stabilise the positively charged carbon atom is preferred cis to the 2-phenyl ring in the alkene product.