Batch synthesis of sulphobutyl ether β-cyclodextrin (also known as SBE-β-CD or SBECD) is a process effectively divided into three main stages, i.e. initial reagent dissolution, a sulphoalkylation reaction and final reaction quenching. This reaction is followed by downstream processing and purification, and ultimate isolation of the solid SBECD material. However, a feature associated with using this synthetic method is that a high proportion of lower substituted SBECD is observed. There is therefore a need to provide an improved synthetic method for producing higher substituted cyclodextrins. The authors here present a Continuous Tank Reactor (CTR) method for preparing sulphobutyl ether-cyclodextrins. The method comprises first contacting cyclodextrin with a base to form activated cyclodextrin. The method then involves separately contacting the activated cyclodextrin with an 1,4-butane sultone to form sulphoalkyl ether-cyclodextrin. The activation reaction is carried out in batch synthesis mode and the sulphoalkylation reaction is carried out under continuous flow conditions resulting in a novel method for the synthesis of highly derivatised cyclodextrins. The work is particularly concerned with producing controlled substitution in sulphobutyl ether β-cyclodextrins and novel compositions of highly substituted sulphoalkyl ether β-cyclodextrins are described.