MAX phases (M = early transition metal; A = Al, Si, Ga; X = C, N) are a large class of 3D layered ternary compounds with the stoichiometry Mn+1AXn (1 £ n £ 3). MXenes are a similarly diverse class of 2D compounds derived from MAX phases via etching of the A-site atoms. It is wellestablished that MXenes are surfaceterminated, and thus better represented by the formula Mn+1XnTx (T = termination), however the nature of the surface composition, coordination, and spatial arrangements are exceptionally challenging to ascertain (Figure ). Both material families have extremely diverse applications: MAX phases are used for industrial components such as heating elements and gas burner nozzles with high temperature applications and MXenes are being tested as functional materials for applications in energy storage, 7,8 hydrogen 9,10 and oxygen evolution catalysis, cancer therapeutics, electromagnetic interference shielding, and wireless communication . The electronic and functional properties of MXenes depend strongly on the surface terminations, so a deeper understanding of the surface chemistry is required to optimize the performance in these applications. As the first reported MXene, Ti3C2Tx has been the most thoroughly studied member of this family, while many other MXenes have been, and continue to be, synthesized and applied to diverse functions. In this work, on the other hand, we focus on the less studied 4d transition metal-based niobium MAX and MXene phases including Nb4AlC3, Nb4C3Tx, Nb2AlC, and Nb2CTx, and Nb2SnC. While Nb2AlC 21 and Nb2SnC were reported in the 1960s, it was not until the 2000s that Nb4AlC3 emerged via a synthesis strategy borrowed from the synthesis of the Ta MAX series. The syntheses of Nb2CTx and Nb4C3Tx MXenes were then reported in 2013 and 2014, respectively, by selective etching of Al from Nb2AlC and Nb4AlC3, following the same methodology as for Ti3C2Tx. Neither the intermediate compound Nb3AlC2