The solid electrolyte interphase (SEI) remains a central challenge to lithium-ion battery durability, in part due to poor understanding of the basic chemistry responsible for its formation and evolution. In this study, the SEI on a non-intercalating tungsten anode is measured by operando neutron reflectometry and quartz crystal microbalance. A dual-layer SEI is observed, with a 3.7 nm thick inner layer and a 15.4 nm thick outer layer. Such structures have been proposed in the literature, but have not been definitively observed via neutron reflectometry. The SEI mass per area was 1207.2 ng/cm2, and QCM provides insight into the SEI formation dynamics during a negative-going voltage sweep and its evolution over multiple cycles. Monte Carlo simulations identify SEI chemical compositions consistent with the combined measurements. The results are consistent with a primarily inorganic, dense inner layer and a primarily organic, porous outer layer, directly confirming structures proposed in the literature. Further refinement of techniques presented herein, coupled with additional complementary measurements and simulations, can give quantitative insight into SEI formation and evolution as a function of battery materials and cycling conditions. This, in turn, will enable scientifically-guided design of durable, conductive SEI layers for Li-ion batteries for a range of applications.