A new method of measuring diffusion coefficients using a Stokes cell and a pH-stat is described. Results for the diffusion coefficients of 10 carboxylic acids are reported. The mobilities of an acid and its anion are found to be similar, and the diffusion coefficients of the different acids are shown to be mainly a function of their molecular volumes. Experiments carried out before the steady state in the diffusion cell is established are in agreement with theory and confirm the geometrical parameters of the porous disc.