The present study was undertaken in order to define kinetics of the initial stage of nonenzymatic browning reaction in model systems of D(+)-glucose and isomers of aminobutyric acids. The kinetics of free radical formation at the initial stage of nonenzymatic browning reaction in examined model systems were determined using Electron Paramagnetic Resonance (EPR) spectroscopy. Approximate rate constants, k1 and Kst, for the induction period and steady-phase of the reaction, were determined as 1.98 × 106s−1 and 3.03 × 106s−1, 1.61 × 106s−1 and 2.74 × 106s−1, and 2.98 × 106s−1 and 4.18 × 106s−1, for the model systems of D(+)-glucose and 2-, 3-, and 4-aminobutyric acid, respectively. In all systems evaluated, the Arrhenius activation energies (Ea) for the two reaction steps were found to be 6.03 × 104 Jmol−1 and 9.22 × 104 Jmol−1 5.05 × 104 Jmol−1 and 7.97 × 104, and 7.22 × 104 Jmol−1 and 11.35 × 104 Jmol−1. Both steps for all model systems had negative entropys of activation, and were found to be –18.6 J/deg/mol and —6.3 J/deg/mol, –14.7 J/deg/mol and –5.1 J/deg/mol, and –22.5 J/deg/mol and –9.6 J/deg/mol, respectively. The overall rate expressions are: (1) (2) for the induction step, and steady-phase step of reaction, respectively. Based on the kinetic behavior of free radical formation and analyses of EPR spectra, it is concluded that formed free radicals are responsible for the pyrazine formation at the initial stage ofnonenzymatic browning reaction, when the reaction between aldoses and amino acids was performed in alkaline water solutions.