Kinetics of Fenton (Fe(II) + H 2 O 2 ) p-nitrophenol degradation reveal that the measured pseudo first-order rate constant is given by the expression: kobs), where K= 20.3 x 10 2 M -1 is the equilibrium constant of formation of the complex Fe(II)-(H 2 O 2 ) n and k= 1.5 x 10 -2 s -1 is the first order rate constant of degradation of the complex to yield Fe (III), OH .and OH -.When oxalic acid is added, the complex Fe(III)-oxalic acid-H 2 O 2 is formed producing a highly oxidized Fe(IV) species as the primary oxidant agent.