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,). 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. (C) 2002 Published by Elsevier Science B.V.