The chlorine electrode reaction (ClER) on Ti/RuO2 electrodes was studied by means of a non-conventional methodology based on the use of the polarisation resistance (Rp). The results obtained in experimental measurements carried out in chlorine saturated NaCl-HCl solutions are presented in this first part. It was verified that only in a limited range of pH the ClER is the main electrode reaction. In this domain, the polarisation resistance of this reaction was measured varying the activity of chloride ions (a(Cl-)(o*)), the partial pressure of chlorine ((p) over bar (Cl2)) and pH (or protons activity a(H+)(o*)). A linear dependence between Rp and a(H+)(o*) was observed, which was explained on the basis of a surface model taking into account the inhibition of active sites by protons. This model leads to an expression for the variation of Rp on pH. which allows obtaining the intrinsic polarisation resistance (Rp(o)), which can be used for the analysis of the kinetic mechanism.