The invention provides a method for preparing sulphoalkyl ether-β-cyclodextrin. The method comprises first contacting cyclodextrin with a base to form activated cyclodextrin. The method then comprises separately contacting the activated cyclodextrin with an alkyl sultone to form sulphoalkyl ether-β-cyclodextrin. The activation reaction is carried in batch and the sulphoalkylation reaction is carried out under continuous flow conditions.