A general method to evaluate the absolute uncertainties in temperatures derived using chemical thermometry is developed and applied to the retro Diels-Alder reaction of cyclohexene. Experiments from previous studies of this reaction are reanalyzed to establish the minimum absolute uncertainty limit. Chemical thermometry results are compared with thermocouple measurements in experiments performed in a flow reactor at 6.1 atm pressure and at temperatures from 957 to 978 K . Using conservative uncertainty estimates, our analysis yields absolute (1 sigma) uncertainties of temperature through chemical thermometry using this reaction greater than +/- 20 at 1000 K. Neither more refined experimental techniques nor computational theory is likely to refine rate correlation parameters sufficiently to reach the absolute temperature uncertainties often reported in the literature for chemical thermometry using the retro Diels-Alder reaction of cyclohexene. Published chemical thermometry uncertainty estimates typically have not quantitatively considered the absolute uncertainties of the original data from which the reference rate correlations were based.