An electrochemical cell with beryllium X-ray windows has been designed and used for in situ small-angle;X-ray-scattering studies of operating electrodes for the first time. This cell is ideally suited to the study of the filling of nanoscopic pores in solids bg electrochemically transported atoms. The mechanism of electrochemical lithium and sodium insertion in nanoporous carbonaceous materials has been the subject of some recent controversy, which is resolved by the studies reported here. We show indisputable evidence that the filling of the pores accounts for all the sodium land lithium) inserted into these carbon, at a chemical potential near that of metallic sodium (or lithium). At lower chemical potential (higher cell voltage), sodium (or lithium) is inserted between graphene layers in an intercalation mechanism.