Changes in mean body temperature (T_b) estimated by the traditional 2-compartment model of ''core'' and "shell" temperatures and an adjusted 2-compartment model incorporating a correction factor, were compared to values measured by whole-body calorimetry. Sixty participants (31M, 29F) cycled at 40% VO_2peak for 60 or 90-min in the Snellen Calorimeter, at 24°C or 30°C. The "core" compartment was represented by esophageal (T_es), rectal (T_re) and aural canal (T_au) temperature, and the "shell" compartment was represented by a 12-point mean skin temperature (T_sk). Using T_re and conventional core-to-shell weightings (X) of 0.66, 0.79 and 0.90, mean T_b estimation error for the traditional model was -95.2% [-83.0,-107.3] to -76.6% [-72.8,-80.5] after 10-min, and -47.2% [-40.9,-53.5] to -22.6% [-14.5,-30.7] after 90-min. Using T_re, X=0.80 and a correction factor (X₀) of 0.40, mean T_b estimation error for the adjusted model was +9.5% [+16.9,+2.1] to -0.3% [+11.9,-12.5] after 10-min, and +15.0% [+27.2,+2.8] to -13.7% [-4.2,-23.3] after 90-min. Quadratic analyses of calorimetry Tb data was subsequently used to derive best-fitting values of X for both models and X₀ for the adjusted model for each measure of "core" temperature. The most accurate model at any time point or condition only accounted for 20% of the variation observed in Tb for the traditional model and 56% for the adjusted model. In conclusion, throughout exercise the estimation of T_b using any measure of "core" temperature together with mean skin temperature irrespective of weighting is inaccurate even with a correction factor customized for the specific conditions.