A gamma thermometer (GT) suitable for very high gamma heating levels (up to 20 W/g) has been designed and modeled by means of detailed finite element calculations. Based on a sensitivity analysis, the predicted accuracy of this GT is better than 5%. Experimental verification of the time constants has been performed, showing excellent agreement. A novel miniaturized GT in which a single thermocouple is used as the gamma absorption element, allowing a reduction of the sensor diameter down to 3 mm, is proposed. The sensitivities of the GTs with stainless steel, W, Mo, and Rh as the heated materials have been modeled by finite element calculations. It is found that both the Mo- and the Rh-based sensors have a very linear response up to a nuclear heating of 20 W/g. Monte Carlo calculations [by Monte Carlo N-Particle (MCNP)] have been performed to assess the relative contribution of neutrons to the nuclear heating in a GT. Calculations have been performed for GTs with an inner body made of various materials, such as stainless steel, tungsten, molybdenum, and rhodium. By using GTs made of different materials, it will be possible to deduce the nuclear heating rates in these materials and also to separate out the neutron and gamma heating contributions. The Monte Carlo calculations show that nuclear heating of rhodium is mainly due to neutrons, converting the rhodium GT effectively in a neutron thermometer.