A recent paper has reported the results of an inter/intralaboratory study into a test and reference reaction for isothermal microcalorimeters, the imidazole catalysed hydrolysis of triacetin. The values derived were 2.8 x 10(-6)+/-9.7 x 10(-8) dm(3) mol(-1) s(-1) and -91.7+/-3 kJ mol(-1) for the rate constant and enthalpy, respectively. This paper reports the findings of an investigation, using this test reaction, into the effect of ampoule fill volume on the recovery of the derived rate constant and enthalpy. Fill volume is likely to be of significance for all commercially available, heat conduction, isothermal microcalorimeters. Experiments, reported here, were performed in a thermal activity monitor (TAM, thermometric AB, Jarfalla, Sweden) operating in the batch mode, for 3, 4 and 20 ml glass ampoules and 4 ml stainless steel ampoules. It is shown that the rate constant can be accurately recovered at fill volumes greater than 50% but below 50% fill volume this accuracy diminishes.