This work is first intended to optimize the experimental conditions for the maximum degradation of guaiacol (2-methoxyphenol) by Fenton's reagent, and second, to improve the process efficiency through the use of solar radiation. Guaiacol is considered as a model compound of pulp and paper mill effluent. The experiments were carried out in a laboratory-scale reactor subjected or not to solar radiation. Hydrogen peroxide solution was continuously introduced into the reactor at a constant flow rate. The kinetics of organic matter decay was evaluated by means of the chemical oxygen demand (COD) and the absorbance measurements. The experimental results showed that the Fenton and solar photo-Fenton systems lead successfully to 90% elimination of COD and absorbance at 604 nm from a guaiacol solution under particular experimental conditions. The COD removal always obeyed a pseudo-first-order kinetics. The effect of pH, temperature, H(2)O(2) dosing rate, initial concentration of Fe(2+), and initial COD was investigated using the Fenton process. The solar photo-Fenton system needed less time and consequently less quantity of H(2)O(2). Under the optimum experimental conditions, the solar photo-Fenton process needs a dose of H(2)O(2) 40% lower than that used in the Fenton process to remove 90% of COD.