The mathematical investigation presented in this article with the behavior of free viscous liquid sheets. The aim of this work is to show the competition among the aerodynamic, surface tension, and viscous forces in the disintegration process, and to deduce the situations in which the liquid viscosity introduces nonnegligible effects. The approach is based on linear theory considerations. The dispersion diagrams obtained show that the viscosity may have a nonneligible effect on the growth of sinusoidal perturbations. However, it is seen that the viscosity has no influence On the cutoff number of the dispersion diagram. The importance of the viscous effect is found to be strongly related to the values of the other parameters. A detailed analysis yields that any liquid sheet may be considered as inviscid if M, defined by M = (mu(L) rho(G)(2)U(3)h)/(rho(L) sigma(2)), is less than 0.001.