(T)he main object of this work was the conception and the building of an experimental device aiming at the study of chemical reactions in liquid medium under stringent monophasic conditions. Thus an isobar isotherm reactor has been made. Its original feature consists of a compensating piston which allows reagents injection and sample withdrawal without appearence of gaseous phase. The method has been successfully tested in the case of the oxidation of the monomethylhydrazine in aqueous medium. The stoichiometry of the reaction has been established.