The jet-cooled dimer of 4-amino-N-methylphthalimide (4ANMP) has four NH bonds, but the ground-state infrared spectrum shows just two strong N-H-stretch resonances in the range 3100-3800 cm - 1 . This confirms a symmetric structure (C 2 ). In the excited state, there are six strong resonances in the same region, falling into two groups of three. A similar pattern is observed for the dimer of 4-aminophthalimide (4AP). These observations suggest that double proton transfer may be occurring across the two N-H...O hydrogen bonds.