A method is described to measure the temperature field of axisymmetric laminar thermals, plumes, and starting plume caps in a liquid. The sample liquid is placed in a Mach-Zender interferometer, and an infinite-fringe interferogram is recorded on photographic film. By comparison with a bar pattern interferogram, the fringe shift field was measured to an accuracy of ±0.02. The Bockasten series approximation of the Abel transformation was applied to the fringe shift field to obtain the radial refractive index variation field, which in turn could be related to the temperature variation field through an empirical relationship. Temperature accuracies in the range of ±0.05 to ±0.5°C were obtained. The radial temperature profile of the plume was found to be monotonically decreasing, while the cap had a local maximum offset from the axis.