The radical scavenger 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO • ) and the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were used in con-junction with mass spectrometry to identify the protein-based radical sites of the H2O2-tolerant ascorbate peroxidase (APX) of the red alga Galdieria partitaand the H2O2-sensitive stromal APX of tobacco.