Laboratory measurements on the temperature field structure in rising turbulent thermals are presented. The relatively small size (about 10 cm initial diameter), but strongly heated thermal bubbles are generated by pulsed arc discharges of about 103 J energy in otherwise undisturbed air. Temperature field measurements are made by stationary and rotating fine‐wire thermometers which probe through the rising bubble. The thermal front is found to be sharp at the top and around the side of the bubble, but become diffused near the bottom, suggesting thermal wake formation. The large amplitude fluctuation, strong gradient, and rich harmonic content of the interior temperature field indicate that the mass entrainment process penetrates deeply into the bubble, and turbulent mixing proceeds at all depths. By invoking passive scalar mixing theories and assuming local equilibrium spectral transfer, the averaged turbulence kinetic energy dissipation rate within the bubble ϵ can be estimated from the diffusive cutoff wavenumber apparent in these preliminary experiments to be of the order of 0.03 (U3/D), where U and D are, respectively, the instantaneous rise velocity and mean diameter of the bubble.