The temperature dependence of internal conversion in model compounds of the chromophore of the green fluorescent protein and one of its mutants has been measured. The strong temperature dependence persists in all charge forms of the model compounds, in all solvents and in a polymer matrix. The ultrafast internal conversion mechanism is thus an intrinsic property of the chromophore skeleton, rather than one of a specific charge or hydrogen-bonded form. An isoviscosity analysis shows that the coordinate which promotes internal conversion is essentially barrierless at room temperature. At reduced temperatures (or high viscosity) there is evidence for the formation of a small barrier. This may reflect a change in the nature of the microscopic solvent dynamics close to the glass transition temperature. In all cases the viscosity dependence of the rate constant for internal conversion is very weak, being approximately proportional to viscosity raised to the power of 0.25 +/- 0.06. This suggests weak coupling between the relevant coordinate and macroscopic solvent viscosity. It is suggested that a potential candidate for the coordinate which promotes internal conversion is the volume-conserving "hula twist".