Processes for preparing polyimides from bis-maleimides are provided. In these processes, a bis-maleimide is polymerized in the presence of a catalytic amount (usually about 0.01 to 5% by weight of the bis-maleimide) of an alkali metal salt of a Bronsted acid. Preferably, the polymerization is also conducted in the presence of a small amount (usually 0.01 to 5% by weight of the bis-maleimide) of an activator for the catalyst. The activator is an aprotic organic compound having a dielectric constant between about 35 and 45. Melt polymerization is preferred at a preferred temperature of 150.degree. to 275.degree.C.