Peroxynitrite (ONOO-), which is produced in vivo by the reaction of nitric oxide (.NO) with superoxide (O2-.), is a selective oxidant. Peroxynitrite decomposes to form nitrate at neutral pH and the decomposition is accelerated by CO2, which is abundant in biological systems, but the mechanism of this CO2-assisted decomposition of peroxynitrite is not fully understood. Previously we showed that CO2 recycles and is a true catalyst in the reaction. In this work, we probed the reaction of peroxynitrite with a limiting amount of CO2 at pH 7.4 and 25 degrees C in the presence of tyrosine. Tyrosine does not react directly with peroxynitrite, but it is able to trap intermediates that arise from the CO2-peroxynitrite reaction. The addition of tyrosine to the CO2-peroxynitrite system results in a reduced rate of decay of peroxynitrite. This effect is easier to identify in later stages of the reaction, and therefore we have used the lifetime of peroxynitrite (the time it takes for the concentration of peroxynitrite to fall to 1/e of its initial value) to characterize the reaction. The values of the lifetime of peroxynitrite rapidly reach a plateau with increasing concentrations of tyrosine. However, even at the plateau value, the lifetime of peroxynitrite still is much shorter than that observed in the absence of added CO2. To rationalize these effects, we suggest that tyrosine partially traps intermediates that arise from the CO2-peroxynitrite reaction, causing the fraction of the CO2 that recycles to decrease and the lifetime of peroxynitrite to increase. Possible intermediates involved in the CO2-peroxynitrite reaction that could be trapped by tyrosine are discussed.