Chemical reaction kinetics for natural materials (e.g., fossil fuels) can often be accurately described by models using a distribution of activation energies. To investigate the general applicability of simpler models, reaction rates and fractions reacted at various heating rates or constant temperatures are calculated using an assumed average activation energy (E) with Gaussian distributions of various widths (< = 0-16% of E). Then the reaction rates or the integrated reaction rates are fit to simpler expressions. Effective kinetic parameters are presented for nonisothermal and isothermal conditions. Two methods are discussed for determining the actual activation energy distribution parameters (A, E, and <). The most general method is multiple nonlinear regression of reaction rates or integrals at two or more heating rates. A faster method utilizes linear regression to obtain E and an approximate value of A, followed by the determination of < and a more accurate value of A from simple correlations developed from model calculations. Finally, effective kinetic parameters for a reaction that has a distribution of activation energies depend on the extent of prereaction (e.g., maturity of petroleum source rocks). Calculated reaction rates are analyzed to determine effective parameters for prereactions of 0-90%. The effective activation energy of the remaining material increases with maturity from an artificially low value to approximately the true average value at high maturity. We also present correlations of T/sub max/ (temperature at which the reaction rate is maximum for a constant heating rate process) with sigma/sub e/ and maturity. 28 references, 11 figures, 4 tables.