MACINNES, Shedlovsky, and Longsworth1claim to have obtained accurate figures for the limiting mobilities of certain monovalent ions,inter alia, K+, 73.50; Na+, 50.10; H+, 349.72; C1−, 76.32; CH3COO−, 40.87 ; and they refer to the discrepancies between these and the older figures compiled by Noyes and Falk.2We would point out that all these constants, except that for the acetate, were determined by us some time ago,3and were based on the use of the Ferguson and Vogel method4for extrapolation to infinite dilution and a value of 0.490 for the cation transport number for potassium chloride ; our figures are K+, 73.4 ; Na+, 49.8 ; H+, 348.0 ; Cl−, 76.4, that for the hydrogen ion being based on conductivity measurements of iodic, hydrochloric, and benzenesulphonic acids in dilute solution in silica cells.We have recently carried out conductivity measurements on sodium and potassium acetates and have corrected the results for hydrolysis by a method which is of general application; our figure for the mobility of the acetate ion is 37-85. New determinations of the conductivity of acetic acid over the concentration range 0-0001 - O-QIN in silica cells of the Hartley and Barrett type gave the value of 1-776 x 10~5 for the true or thermodynamic dissociation constant of this acid. This is much higher than the figure 1-753 x 10"5 obtained by Maclnnes and Shedlovsky.5 Full details of these results will be published in the near f