A method is proposed for the separation of all the components of a complex mixture of acids or bases by counter-current distribution with progressive variation of the pH. The function that controls the double distribution and dissociation equilibrium in the case of a weak base was investigated. Since the separation depends on the difference in the product of the dissociation constant Kb and the distribution coefficient Kr, two possibilities are considered; the use of a lower buffer phase and an upper organic phase whose composition is progressively changed in such a way as to vary Kr, the bases being eluted in order of increasing Kr·Kb, and the use of a lower organic phase with an upper buffer phase whose pH is varied from neutrality to increasingly acidic values in such a way as to extract the alkaloids in order of decreasing Kr·Kb. With the aid of this second process, it was possible to isolate the known nine alkaloids of Strychnos nux-vomica, i.e. strychnine, α- and β-colubrines, brucine, pseudostrychnine, pseudobrucine, icajine, vomicine, and novacine, as well as four others that had not been discovered previously. The separation of strychnine and brucine on the basis of the difference in the product Kr·Kb was also examined. Where this product was equal (e.g. for colubrines and brucine between chloroform and water), partial modification of one phase (addition of 35% of ethyl acetate) leads to non-proportional changes in the Kr values, and so permits separation. © 1969.