Efficient base-catalysts for low temperature solid-phase imidization of poly(amic acid) (PAA) have been found. Thermal imidization of PAA proceeded smoothly in the presence of 1 wt% of strong bases such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at 200degreesC in a short time, and no residues of DBU remained in the polyimide film.