We investigated the solidification rate of polyvinylidene fluoride (PVDF) membranes during preparation by the nonsolvent-induced phase separation (NIPS) method. A new apparatus for quantitatively measuring membrane stiffness during phase separation was developed. In this apparatus, a polymer solution placed on a stage moves upward and the surface of the polymer solution contacts a sphere attached to the top of a needle. The displacement of a blade spring attached to the needle is then measured by a laser displacement sensor and converted to the surface repulsive force of the polymer solution. The effects of polymer concentration, composition of coagulant, molecular weight of polymer, and addition of hydrophilic additives were investigated. The solidification rate increases with increasing polymer concentration and molecular weight and decreases with the addition of solvent to the coagulation bath. The addition of hydrophilic additives causes rapid uptake of nonsolvent into the polymer solution and had the largest effect on solidification rate. Based on these results, two correlation groups between the solidification rate of the polymer solution and the mechanical strength of the ultimate membrane were identified. This is the first work to directly and quantitatively measure the solidification rate during nonsolvent-induced phase separation. (C) 2013 Elsevier B.V. All rights reserved.