A modified screen printed electrode with [Fe(tris(3,5–dimethyl–1–pyrazolyl)borate)2]+[FeCl4]–, installed as part of a flow injection system, was used as an amperometric detector to determine hydrogen peroxide and glucose after on–line reaction with glucose oxidase. The influence of experimental variables on sensitivity, such as the flow rate, injection volume and reaction coil length, was evaluated using a two–level factorial experimental design. Under optimal conditions, detection limits of 15 μM and 0.5 mM for H2O2 and glucose, respectively, were obtained. A sampling rate of 60 determinations h–1 was achieved; the relative standard deviation of analytical measurements was <5.0%. The method was validated by comparing the obtained results with those provided by the enzymatic spectrophoto–metric method; no significant differences were observed (p < 0.05).