Observations of the Martian planetary boundary layer lead to interpretations that are baffling and contradictory. In this paper we specifically address the question of whether or not water vapor finds a substantial diurnal reservoir in the Martian regolith. To address this issue, we have measured H2O adsorption kinetics on SWy-1, a Na-rich montmorillonite from Wyoming. The highest-temperature (273 K) data equilibrate rapidly. Data gathered at realistic H2O partial pressures and temperatures appropriate to early morning show two phenomena that preclude a significant role for smeclites in diurnally exchanging a large column abundance. First, the equilibration timescale is longer than a sol. Second, the equilibrium abundances are a small fraction of that predicted by earlier adsorption isotherms. The explanation for this phenomenon is that smeclite clay actually increases its surface area as a function of adsorptive coverage. At Mars-like conditions we show that the interlayer sites of smeclites are likely to be unavailable. ¿ 2001 American Geophysical Union