A study of the breakup of planar viscous liquid sheets subjects to gas flow on both sides was conducted. A linear spatial stability analysis was used to determine the instability wave characteristics. The analysis included the effects of liquid properties such as viscosity, density, and surface tension; the gas was treated as inviscid. Dispersion relations were obtained relating the wave growth rates to the frequency and other flow variables. The wave characteristics were determined by numerical solution of the governing dispersion relations for a wide range of operating conditions. In all cases, the gas velocity was found to be destabilizing; increases in the liquid density, viscosity, and surface tension were all found to have stabilizing effects. When the liquid sheet was exposed to unequal gas velocities, the wave propagation characteristics were found to be altered from the case of equal gas velocities.