A new distributed activation energy model (DAEM) is developed using Weibull distribution to mathematically represent the kinetics of complex thermal transformations. Also developed is a grid search procedure to estimate the kinetic parameters represented by the DAEM. This procedure uses approximate analytical solutions to the time-temperature integrals of DAEM. These approximations reduce the computational efforts involved in parameter estimation substantially and allow rapid analysis of the three-dimensional error surface of the objective function over a wide range of kinetic parameters while retaining the true nonlinearity of the model. The existence of multiplicity-a number of sets of kinetic parameters which describe the observed data, generally within the limits of experimental accuracy-is also demonstrated using experimental data on kerogen decomposition. Several sets of isothermal and nonisothermal experimental data are used to validate the DAEM. This work seems to be the first published one where Weibull distribution is used to describe the kinetics of complex thermal transformations.