The authors have investigated inactivation kinetics of mixed microbial populations, which consist of component subpopulations following first-order inactivation kinetics. Populations which had undergone previous partial heat treatment were also included in the study. As the survivor ratio is the Laplace transform of the rate constant distribution an analysis of survival functions following nth-order inactivation kinetics (n > 1) resulted in a gamma distribution of the first-order rate constants. The expected value of the gamma distribution is proportional to the resulting nth-order rate constant and inversely proportional to the nth-order time constant. According to the theory, the nth-order time constant and survivor ratio are independent of the initial microbial concentration. The temperature effect has been treated with the restriction that a monotonously increasing functional relationship exists between first-order rate constants at two different temperatures. This monotonous function relates first-order rate constants of the same subpopulation. The Laplace transformation method is applicable to create analytical functions for fitting them to measured semilogarithmic survivor curves, if these curves are of the concave type.