  In this paper we consider a generalized Kuramoto-Sivashinsky equation. The
equivalence group of the class under consideration has been constructed. This
group allows us to perform a comprehensive study and a clear and concise
formulation of the results. We have constructed the optimal system of
subalgebras of the projections of the equivalence algebra on the space formed
by the dependent variable and the arbitrary functions. By using this optimal
system, all nonequivalent equations admitting an extension by one of the
principal Lie algebra of the class under consideration can be determined.
Taking into account the additional symmetries obtained we reduce some partial
differential equations belonging to the class into ordinary differential
equations. We derive some exact solutions of these equations.
