Calorimetry is a technique for measuring the thermal power of heat producing samples and it is widely used in a variety of measurement fields including chemical energy measurement, nuclear half-life determination and nuclear materials accounting and safeguards1, 2. The technique has specific application in measuring the heat produced by the radioactive decay of tritium bearing materials. This paper describes the design and proposed operation of a transportable isothermal tritium calorimeter for use as part of the measurement instrumentation of the JRC Ispra European Tritium Handling Experimental Laboratory (ETHEL). An instrument based on this design is currently under construction. The advantages of the use of the instrument are discussed as are the issues of safety, measurement accuracy, measurement time and ease of use. Data are presented from measurements of low power plutonium samples (which simulate tritium in a uranium bed) using a plutonium calorimeter based on similar design concepts. The measurements correspond to tritium quantities ranging from less than 1 kCi (37 TBq) up to greater than 100 kCi (3.7 PBq or 10 grams).