Abstract The barrier shapes of a series of doped tunnel junctions of the general form A1-I(X)-M have been determined using a three parameter (φ1, φ2 and d) trapezoidal analysis of their observed I–V characteristics. The parameter φ2 represents the step between doped oxide layer and the metal M (= Pb or Ag), and d the effective barrier thickness. The adsorbates are a set of 4-substituted benzaldehydes chemisorbed, following surface reaction on the insulating oxide layer, as the corresponding carboxylate anions. Correlations are found between the fitted values of φ2 and the commonly recognised Hammett substituent parameters σ and σR and the pKa values of the related substituted benzoic acids as well as the calculated molecular dipole moments of the derived carboxylate anions. These imply prescriptive possibilities in the choice of adsorbates which may show enhanced Raman responses.