For a number of polyvinyl alcohol (PVA) samples polymerized at 60°C., viscosities of their concentrated aqueous solutions were measured over the ranges of concentration from 1% to 25% ( w w ) and of temperature from 30°C. to 80°C. It was found that: ( 1) The relative viscosity may be expressed in terms of a single function of ( C 2 5 M w 3.4) over the range of C 2 5 M w 3.4 from 1.5 × 10 11 to 1.6 × 10 13 at 30°C.; this range is temperature dependent. ( 2) The critical value of V 2 Z w , which is interpreted as the onset of a continuous network with ξ = 2, changes from 200 at 30°C. to 240 at 80°C., where V 2 is the volume fraction of polymer in solution, Z w is the weight-average chain length, and ξ is the average number of entanglement points per molecule. Ferry's empirical law, η r ∝ C 2 5 M w 3.4, appears to hold in the range 2 < ξ < 5 for PVA and many other polymers (polystyrene, polymethyl methacrylate, and cellulose derivatives), except for polyisobutylene, for which the glass transition temperature is very low compared with that of other polymers. ( 3) The concentration dependence of activation energy for viscous flow of PVA is similar to that of cellulose derivatives. ( 4) In the low molecular weight region ( M < M crit), the molecular weight dependence of viscosity varies with temperature and concentration.