Nonstandard finite difference schemes are considered for a generalization of the Cahn-Hilliard equation with Neumann boundary conditions and the Kuramoto-Sivashinsky equation with periodic boundary conditions, which are of the type $$Ut;+;frac{{partial}^2}{{partial}x^2} g(u,;Ux,;Uxx);=;frac{{partial}^{alpha}}{{partial}x^{alpha}}f(U,;Ux),;{alpha};=;0,;1,;2$$. Stability and error estimate of approximate solutions for the corresponding schemes are obtained using the extended Lax-Richtmyer equivalence theorem. Three examples are provided to apply the nonstandard finite difference schemes.