We present an analysis of Nb 3 Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb 3 Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ±0.4 meV and superconducting critical temperatures (T c ) up to 16.3K. Scanning transmission electron microscopy (STEM) performed on cross sections of the sample's surface region shows a ∼ 2 microns thick Nb 3 Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb 3 Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low T c regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb 3 Sn -coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.Discovered in 1954 1 , the A-15 compound Nb 3 Sn is a Type II (κ∼20) strong coupling s-wave superconductor 2,3 with a maximum T c of 18 K 4 and superconducting order parameter ∆ of 3.4 meV 5 . Due to its relatively high T c and ability to carry high current densities, Nb 3 Sn is an ideal candidate for replacing NbTi for superconducting wire applications and Nb for superconducting radio frequency (SRF) resonators operating from a few hundred MHz up to several GHz. Early work into developing Nb 3 Sn for SRF applications started in the 1970s 6-9 . In particular, researchers from Wuppertal University optimized a coating recipe 8 based on the diffusion of Sn vapor into elemental Nb at temperatures between 1000 • C to 1200 • C. This approach has the unique advantage of being scalable to applications for which a coating process without a direct line of sight is required. State-of-the-art RF performance tests then 10 showed an extremely high quality factor ∼ 10 11 at 2K and ∼ 10 10 at 4.2K (about 20 times higher than pure Nb) with a strong decrease of the quality factor (Q -slope) above an accelerating field of 5 MV/m. The origin of this Q -slope remains unclear, however it was postulated that the onset of the Q decrease at 5 MV/m (peak surface magnetic field of 22 mT) was due to early vortex penetration above the Nb 3 Sn first critical field B C1 , and therefore was an intrinsic material limitation. A regain of interest was stimulated by recent RF tests done at Cornell University 11 that reproducibly exhibit a similar Q factor ∼ 2 × 10 10 at 4.2K (and 3 × 10 10 at 2K), but a very moderate Q -slope up to a quenching field of 12-17 MV/m, corresponding to a a) Electronic mail: prolier@anl.gov peak surface magnetic field of 50-70 mT, which is significantly higher than the B C1 of 25±7 mT measured on this cavity 11 . Another striking and reproducible feature is the very moderate ...