A new in situ wall conditioning method by reactive gas fuelling has been investigated in TEXTOR; this method is applicable during plasma discharges of the operating machine. For these experiments, trimethylboron, B(CH3)3, was fed into the torus instead of hydrogen. While the hydrogen of the trimethylboron molecules fuels the plasma to sustain the discharge, boron and carbon are screened at the plasma edge and redeposited at the wall. The impurity concentration in the plasma is negligible, as it is after the usual boronization by radiofrequency assisted glow discharge. The plasma performance was improved shot by shot. This new technique may open a way to direct local control of erosion and redeposition processes at wall elements subject to high heat loads