The anionic form of p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) has been extensively employed as a model of the chromophore of the green fluorescence protein. The bright S1 excited state HBDI(-) has a measured lifetime of 1.4 ps in the gas-phase and is dominated by two non-radiative decay mechanisms: internal conversion and autodetachment into the neutral continuum. Here, time-resolved photoelectron spectroscopy has been used to determine the yields of these two channels from which the lifetime for autodetachment was found to be ∼30 ps.