The invention provides water swellable wound dressing materials comprising from 5% to 50% of an alginate ester of a C.sub.1 -C.sub.6 polyhydric alcohol; from 50% to 95% of a humectant consisting of one or more C.sub.1 -C.sub.6 monohydric or polyhydric alcohols; and from 0% to 30% of water, the percentages being calculated by weight based on the weight of the material when anhydrous. The preferred alginate ester is propylene glycol alginate (PGA). The materials swell but do not dissolve in cold or warm water. Insolubility is achieved by the addition of polyvaent cations such as calcium ions, or by covalent cross-linking of the pga, or by adding from 10 to 35% by weight of water-swellable polysaccharide such as gelatin, or by adding from 5% to 20% by weight of a cationic polymer such as chitosan.