The sulfonation kinetics of poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbon -1,4-phenylene)(PEEK)has been studied at highly dilute conditions (approximately 0.02 g l-1) in 96.3% sulfuric acid. The sulfonation was observed from 35 to 75-degrees-C using ultraviolet-visible spectroscopy. The fractional sulfonation has been calculated from the ratio, I-I0/I(infinity)-I0, where I, I0 and I(infinity) represent the spectral absorption of the reaction system at a time t, at the starting point and at the ending point, respectively. The I0 is extrapolated from a least-squares plot of the data calculated through a kinetic equation combined with the ratio cited above. The reaction rate was confirmed to be first order in the PEEK chain repeat unit and also inversely first order with respect to the concentration of the sulfonated PEEK repeat unit. The reaction follows the Arrhenius equation over the temperature range of study, with a calculated activation energy of 82.8 kJ mol-1 repeat unit. Good agreement has been obtained between these results and the data measured in a more concentrated system (20 g PEEK/l H2SO4).