We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10−18)X, current densities (J) increases > 4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements shows tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and

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... tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drop at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. We demonstrate experimentally that β ∝1/√εr, suggesting highly-polarizable terminal-atom sites act as charge traps as proposed by Berlin and Ratner. Our work shows the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.