AbstractThe theory of continuous crystallisation (especially of sucrose) in crystallisers connected in series, in which a suspension of growing crystals is fed forward through the system without backmixing, is presented, and the calculation of the minimum coefficient of variation that can be attained under stated conditions is described. Two cases specifically considered are: (1) stirred reactors of equal mean residence times connected in series, and (2) tubular reactors in which the residence time distribution is given by a Gaussian error function. Reduction of coefficients of variation either by connecting reactors in series or by extending a tubular reactor in the axial direction is considered. Series connexion is always superior to extension because it prevents backmixing at the points of connexion.