The reaction between sodium bicarbonate (NaHCO3) and atomic H is the only likely route (apart possibly from daytime photolysis) for recycling this major sodium reservoir back to atomic Na in the upper mesosphere. The rate coefficient was determined for the first time by reacting NaHCO3 in a known excess of H in a fast flow tube, yielding k(227 K)=(1.7¿0.6)¿10-13 and k(307 K)=(5.2¿2.5)¿10-13 cm-3 molecule-1 s-1. These measurements are very well described by transition state theory, using ab initio quantum calculations of the saddle points on the reaction potential energy surface. The resulting temperature dependence of the rate coefficient is then given by k(100--400 K)=1.84¿10-13 T0.78 exp-(1014-96+130/T) cm3 molecule-1 s-1. When k(T) is extrapolated to 180--220 K, there is excellent agreement with the value predicted from atmospheric models of the nighttime Na layer. Hence this measurement has, to a first order, demonstrated closure of the neutral chemistry governing the underside of the Na layer above 80 km. ¿ 2001 American Geophysical Union