Ernest: a toolbox for chemical reaction network theory. Summary: ERNEST Reaction Network Equilibria Study Toolbox is a MATLAB package which, by checking various different criteria on the structure of a chemical reaction network, can exclude the multistationarity of the corresponding reaction system. The results obtained are independent of the rate constants of the reactions, and can be used for model discrimination. Availability and Implementation: The software, implemented in MATLAB, is available under the GNU GPL free software license from∼altafini/papers/SoAl09/. It requires the MATLAB Optimization Toolbox.