University College of Science and Technology, Calcutta-700 009 Manuscript received 25 March 1983, revised 21 September 1983, accepted 11 February 1984 From a consideration of the chemical potential of the ions of a strong electrolyte dissolved in water it has been found that k (kappa) of the Debye.Huckel theory requires modification. Further theoretical considerations have led to the conclusion that the modified kappa, denoted by km, should vary with dilution according to cube-root of concentration. Equations deduced by using this km show that partial molal volume and apparent molal volume of a strong electrolyte in solution should vary with dilution according to cube root of concentration. The equation for the apparent molal volume bas been subjected to test using the data available in the literature on NaCI, KCI, NaBr, NH4N02, Na2S04 and SrCI2 and has been found satisfactory.