Processes for producing terephthalic acid are disclosed, the processes including a step of combining in a reaction medium para-xylene, a solvent comprising water and a saturated organic acid having from 2-4 carbon atoms, and an oxygen-containing gas, at a temperature for example from about 145°C to about 175°C, in the presence of a catalyst composition comprising cobalt, manganese, zirconium, and bromine, wherein the zirconium is present in the reaction medium in an amount, for example, from 1 ppm to 50 ppm with respect to the weight of the liquid in the reaction medium.