[en] A method is given to reconstruct the local state of a heavy ion collision which produced the detected particles. Only some particle ratios are needed; in the simplest case 2 of them determines the local state if it was in complete thermodynamic equilibrium, and the assumption is checked later. Applying the method on the available data of the E802 experiment, T ∼ 120 MeV, n/n0 ∼ 0.4 and S/NB ∼ 14 is obtained, which is a reasonable break-up stage. (author) 22 refs.; 7 figs