For those microorganisms exhibiting log linear inactivation kinetics, the D and z values describing their inactivation are typically measured in a series of isothermal experiments, each measuring log reductions with time. This paper examines the mathematics of D and z measurement in order to show how both values could be obtained in fewer isothermal experiments than are traditionally performed. Additionally, a similar analysis of a non-isothermal experimental approach shows how even a minimum of 3 data points in one experiment can be used to determine both parameters. This latter approach, though discussed previously in the literature, is given an analysis in this paper which reveals previously unrecognized details of the method and its performance in extracting D and z values from experimental data. Published by Elsevier Ltd.