This study focused on the application of the Response Surface Methodology (RSM) to determine the effects of operational parameters on the dark Fenton oxidation of a barley straw effluent (BSE) in a lab-scale experiment, with a chemical oxygen demand (COD) of 749 mg·L-1 O2 at pH = 3. A central composite face-centered design (CCF) with 17 experiments was adopted for our study, since responses could be simply related to the chosen factor (COD, UV254 or UV280 removal) using linear or quadratic models. This methodology let us find the effects and interactions of the studied variables and their roles on the efficiency of treatment process. In the optimization, the correlation coefficients for the model (R2) were 0.996 for COD, 0.995 for UV254 and 0.994 for UV280. Optimized reaction conditions were established as pH = 3, T = 40 º C, [H2O2] = 1800 mg·L-1 and [Fe (II)] = 250 mg·L-1. Under these conditions; 120 minutes treatment resulted in a 81.6 % COD removal, an 89.1% UV254 removal and 93.0 % UV280 removal