In this work diamino-porphyrin derivatives, in their free base or cobalt complex forms, have been used to construct SMJs. Porphyrin oligomers were covalently bonded to a bottom electrode by diazonium cation electroreduction and a STM tip was used to complete the junction. Conductance versus time (G(t)) measurements reveal stable SMJs with lifetimes as long as 70 s, attributable to the diazonium bonding procedure and the amino anchoring groups. SMJ conductance was studied statistically by three methods: the STM-bj G histo (d) histograms, STM G time (t) and voltage-dependent G volt (V) heat maps. Conductance and attenuation factors (b) for Co or free-base porphyrin SMJs, compared by the three methods, are fully consistent, length-dependent and show a strong molecular signature. Trends in the variation of the attenuation factors versus voltage indicate a voltage-driven b decrease for both types of SMJ.