Knowledge of the origin of quick clay and of secondary changes in strength is of importance for construction purposes as well as for the prevention of landslides. Clays of different sensitivity can occur in the same stratum within a few metres of each other. A detailed study was made of adjacent clays in the same layer and of the same age — but with different sensitivity. Different analyses indicated that the sensitivity depends on chemical conditions in the pore water of the clay. By experiments it was demonstrated that quick clay can be changed to normal clay by replacing its pore water by pore water of normal clay or by artifical solution prepared on the basis of chemical pore water analyses. This was later verified by experiments to study the effect of the addition of different salts on the geotechnical properties of clays.