The production of cellulosic bioethanol, one of the two main possibilities of second generation biofuels, involves four dependent steps: pretreatment, enzymatic hydrolysis, fermentation and distillation. This work concerns the first step whose purpose is to modify the structure of the lignocellulosic biomass hence allowing cellulose to become more accessible to enzymatic hydrolysis. In this paper, the biomass flow is characterized in a pretreatment reactor. This is done by performing residence time distribution (RTD) experiments along with the use of new methodology well adapted to our system, industrial pilot plant presenting tough thermodynamic conditions. In this novel methodology, the tracer, sodium carbonate, reacts with the acidified biomass. Our protocol consists in measuring the variation of electrical conductivity and pH at the reactor entrance and exit. Two experiments were performed in the reactor for two different theoretical residence times. A chemical model was developed allowing the determination of tracer concentrations from the measured data. The experimental residence time distributions in the reactor, obtained by deconvolution, are well represented by using CSTR and axial dispersion models. The obtained results point out the nearly plug-flow behaviour of the pretreatment reactor.