The effectiveness of freezing and thawing as a means of reducing the mean particle size of fine-grained sediments has been investigated. It is shown that the amount of size reduction which takes place depends on the structure of the clay mineral. Where bonding between the basal planes of component sheets of the clay mineral is strong there is little size reduction by congelifraction. Where the bonding is weak expansion of ice produced from water entering between the basal planes causes rapid comminution.