The sensitivity of Leda clay from the Ottawa area is relatively independent of the salt content in the pore water. Sensitivity values may vary between 10 and 1 000 in the low salt content range (less than 2 g/liter); at higher salt contents strong flocculating effects of the electrolyte limit the sensitivity, but values as high as 75 have been measured.A relationship has been determined experimentally between electrokinetic potential and sensitivity that is consistent with the theory of interparticle repulsion and attraction. The main deviations are attributable to differences in grain size.Chemical analyses of pore water show that large variations in sensitivity, particularly within one profile, are related to the nature of the pore water electrolyte in the low salt content range. Soils that are unusually sensitive have a high monovalent cation content. Large differences in sensitivity can be accounted for in this way, but the reason for small variations is still obscure.The electroosmosis technique used to determine electrokinetic potentials is discussed in detail. This apparatus was used also to verify Rosenqvist'a leaching theory of sensitivity for soils known to have had a marine origin.