Bigelow＇s method is the basis of all modem thermal process calculations. These calculations are based on the assumption that the heat inactivation model follows first order kinetics. However as demonstrated in this paper; Bigelow＇s method is applicable in all cases when the heat inactivation process can be characterized by a relation of three variables: t{T,N} = g{T}.f{N}; where t = heat treatment time at constant temperature T; T = temperature; and N = number of surviving micro-organisms at time t. The function g{T} depends only on T and f{N} is another function depending only on N. As shown in this paper the (classic) empirical kinetic equation satisfies this requirement, however several non-linear semilogarithmic microbial heat inactivation models do not conform to the above relationship. In dubious cases, when proper equations for experimentally determined heat inactivation curves are uncertain or unknown, a simple graphical method is proposed to assess whether these curves correspond to a g{T}.f{N} type function.