The Los Alamos National Laboratory tritium processing facility purchased an ANTECH Series P300 Isothermal Tritium calorimeter from the ANTECH Corporation in 2005. The instrument is used to nondestructively measure the quantity of tritium in an item based on the heat output generated by tritium beta decay. Instrument performance data collected over the past two years is presented showing that this instrument can detect as low as 0.001 W (0.003 g of tritium) within rather large measurement canisters of approximately 170 mm (6.8 in.) diameter by 610 mm (24 in.) long. With a manufacturerstated measurement power range of 0.005 W to 15 W, this calorimeter has performed beyond the specified purchase requirements. Using a combination of sensors for its thermal element, the P300 calorimeter has demonstrated exceptional sensitivity and precision.