KinFitSim® 3.0: Kinetics simulation and fitting software. KinFitSim 3.0 is a tool for simulating complex chemical reactions that may be employed to develop a quantitative understanding, optimize reaction conditions in homogeneous reacting systems in liquid or gas phase and even devise a true reaction mechanism based on experimental measurements. Chemical reaction mechanisms with tens of reactions and involving a large number of chemical species are not uncommon in models of such systems. Analyzing experimental data with the aid of numerical simulation may yield important information about system kinetics when kinetic parameters of the model are chosen to give best fit between experimental and model responses. KinFitSim is designed for chemists, biochemists, chemical engineers, physicists and others who want to solve problems of this sort without being concerned with numerical methods. It automatically simulates the time-course of chemical or biochemical reactions given the information about the stoichiometry of the reaction mechanism, kinetics of each reaction and initial concentrations of all species. Simulated data may be plotted in the program window or conveniently exported into other data handling software. This new release of KinFitSim includes a number of enhancements compared to previous versions of the software. These changes significantly improve the convergence of the numerical solution of ordinary differential equation (ODE) systems and fitting, reducing the need for users to specify sufficiently good initial guesses for kinetic parameters to be fitted.