The thermal decomposition of cinnamic, p-coumaric, ferulic and sinapic acids has been investigated by employing TG/DTG/DTA methods in the temperature range at ambient to 500 degrees C in a self-generated atmosphere, It established high thermal stability of the investigated acids. The presence of two methoxy groups in the structure increases the thermal stability of sinapic acid. The thermal decomposition of alpha-tetralyl hydroperoxide (THPO) in the presence of phenylpropionic acids has been investigated under nonisothermal conditions. p-Coumaric acid inhibited THPO thermal decomposition in the system containing 0.006, 0.018 mM acid and 0.427 mM THPO. The increased content of p-coumaric acid (0.049 and 0.06 mM) in the reaction system leads to rapid reaction with tetralylperoxy radicals and the formation of THPO. Cinnamic acid interacts with THPO radicals and suppresses hydroperoxide free-radical-induced decomposition. Participation of cinnamic acid is not registered in the chain-transfer reaction leading to THPO formation (investigated systems -0.427 mM THPO and 0.006-0.06 mM cinnamic acid), Ferulic acid inhibits THPO thermal decomposition in all the mol proportions studied (THPO : ferulic acid 1, 0.012 to 0.180). Ferulic acid is regenerated as a result of the interaction between ferulic radicals and THPO. Low concentrations of sinapic acid inhibit THPO decomposition.