The water adsorption capacity of desiccant materials may be affected significantly if particulates are present in the air that is to be processed. Air containing environmental tobacco smoke was used to study the effect of particulates. A smoking apparatus was designed by simulating the puffing of a cigarette by a person to generate environmental tobacco smoke. The apparatus can take a puff from a cigarette at 1 min intervals; one puff requires 2 s and entails drawing a total of 35 cm(3) of air through the cigarette. The puffing process was controlled by a computer. The sidestream smoke generated during the idle burning of the cigarette in the 1 min interval between puffs was diluted with the room air to produce the environmental tobacco smoke and was used in all the experiments. Desiccants used in the present work included silica gel and molecular sieve 13X. Experiments were carried out under dynamic conditions using a packed bed adsorber at 298 K and at a relative humidity of approximate to 50%. The adsorption and regeneration cycles were repeated 15 times using the same adsorbent. Significant amounts of hydrocarbons and particulates were removed by both silica gel and the molecular sieve during the initial period of a run. The equilibrium water uptake capacity of silica gel at higher relative humidities decreased significantly after 15 cycles but that of the molecular sieve remained almost the same as that of a fresh sample. Also, a molecular sieve that was exposed to tobacco smoke could be regenerated at a lower temperature than the exposed silica gel.