The relative reactivities of cis and trans isomers of β-bromostyrene towards polystyryllithium have been determined by Raman spectroscopy. Polystyryllithium was prepared by the anionic polymerisation of styrene in cyclohexane using sec-butyllithium as initiator and subsequently reacted with mixtures of isomers of β-bromostyrene. The end groups of the resultant polymers were analysed by Raman spectroscopy in the spectral region 1550–1700 cm −1 which contains the bands due to cis and trans ν(CC) stretching vibrations. The cis isomer of β-bromostyrene has been found to be about 14 times more reactive to polystyryllithium than the trans isomer which correlates well with theoretical predictions of lower stability for the cis isomer.