This paper is concerned with an inverse problem of differential equations with special application to determination of parameters expressing a process for petroleum generation accompanying with complicated reactions on kerogen, or multi-component matter. By geochemical theory a total generation rate curve is approximated as a sum of rate curves by first-order reactions parallel to each other, while experimental methodology enables us to observe only the total rate curve for the sampling matter in a laboratory. A method to determine kinetic parameters is proposed by maximization of the cross entropy from a measured rate curve to a space of theoretical curves from a point of probabilistic view. A simple algorithm for the maximization is also proposed, and the perfbrmance is discussed through simulation studies.