We studied electron tunneling across an atomically-flat hBN barrier sandwiched between a graphite electrode and redox levels in a liquid solution. The steady-state voltammetry of [IrCl 6 ] 3- oxidation and [Ru(NH 3 ) 6 ] 3+ reduction obtained on disk ultramicroelectrodes with typical diameters of 5 µm revealed an exponential dependence of the apparent electron transfer rates on the hBN thickness for 1L and 2L hBN. For thicker hBN, deviations in k 0 , j lim , and α (1-α) were observed, indicating limitations of the Butler-Volmer model of electrochemical kinetics. We found that this behavior is a direct experimental manifestation of the Marcus-Hush theory, which has been one of the most fundamental treatments of electron transfer. Furthermore, the experimental platform developed here could be exploited in a range of diverse scenarios, including studies of long-range electron transfer, electrochemical switching between outer-and inner-sphere reaction, and electrocatalytic mechanisms. It could be possible to discriminate between outer-sphere and inner sphere reactions as recently shown for 1L MoS 2 on Au, explore electrocatalytic reactions on hBN/metal heterostructures, and study proton tunneling/transport through 1L hBN. Such developments are even more likely to succeed, once a reliable control over the thickness and quality of synthetically-grown, large-area hBN is established. 62