The distribution coefficients of rubidium and caesium between Cation-42,a phenolic-sulfonated canon exchange resin manufactured in Shanghai,and nitric acid as well as nitric acid-methanol or butanol mixtures have been determined by the equilibrium method.The distribution coefficient gradually increases with increasing methanol content in the solvent.With the addition of butanol,however,the distribution coefficient gradually decreases at first and then raises with further increase of the alcohol content.The favorable effect of lowering the distribution coefficient with simultaneous increasing the separation factor between caesium and rubidium by the addition of 8 per cent butanol is,however,offset partially by the increase of viscosity of the mixed solvent.A tomparatively rapid procedure has been devised,which employs 0.47-0.60 M nitric acid as cluent and a Cation-42 (160-200 mesh) column (0.75×11.5 cm) and separates microamount of rubidium and caesium within 1-2 hours.