Swelling or contraction plays an important role in affecting the mass transfer during osmotic dehydration (OD) of hydrophilic and hydrophobic gels. The initial stage of the OD process is characterized by mass loss until the gel volumes reach a minimal value. Two types of behaviours are observed after this point. The first one (Type I) shows a turning point and re-swelling of the gel. The main reason for such behaviour is the relatively high swelling pressure of the gel in a sugar or salt solution. In certain solutions, such as of sugars and sodium chloride, hydrophilic polyacrylamide gels swell even to a larger extent than in water. The second type of behaviour (Type II) is characterized by total lack of re-swelling capability, which is most likely the result of a phase separation process. In hydrophobic gels, phase transition may take place under critical conditions that are determined by the combined effect of temperature, type and concentration of low molecular solutes. In hydrophilic gels, on the other hand, "salting-out" is the most likely reason for phase separation. The driving forces of all these phenomena are attributed to the effects of the preferential interaction between the low molecular solutes and the gel polymeric matrix.