Two new derivatives of the green fluorescentprotein (GFP) chromophore each containing a charged substituent (CH 2 CH 2 NH 3 + and CH 2 CH 2 NMe 3 + , respectively) on the imidazol-one ring have been prepared. These compounds are model systems for the neutral GFP chromophore with an unprotonated phenol moiety. The ammonium groups allow for electrospray ionization and the possibility to study the optical