The amino acid L-theanine is frequently used in dietary supplements and nutraceutical foods and beverages for its ability to modulate brain activity and neurotransmitter levels. The instability of this compound in acidic formulations (pH 2-3) is concerning and impactful for both product efficacy and shelf-life. This study investigates the thermo-oxidative degradation kinetics of L-theanine over 8 weeks in two formulations (F1 and F2) stored at 4, 25, and 40℃ under low relative humidity (< 40%) and acidic conditions (pH 3.2 for F1 and 2.4 for F2). The degradation rate constant (k), activation energy (E_a), molecular vibration frequency factor (A), and half-life (τ_(1/2)) for each formulation were estimated from concentration-time data using a first-order kinetic model and the Arrhenius equation. Kinetic constants were derived by χ^2minimization, taking into account the measurement errors. The activation energies for L-theanine degradation were 72.5 ± 21.9 kJ/mol for F1 and 105.3 ± 8.6 kJ/mol for F2. The findings provide a mathematical model for predicting the shelf-life of L-theanine-containing formulations under varying storage conditions relatively quickly and cost-effectively, while still in the R&D stage. These insights are valuable for the development of more stable products and can assist in the optimization of manufacturing processes to ensure product efficacy over its intended lifespan.