Palmitoil cellulose Langmuir-Blodgett (LB) films have been prepared on quartz crystal oscillators in the fabrication of novel humidity sensors. The water sorption characteristics of the cellulose LB humidity sensors were investigated quantitatively by the microbalance method using a quartz oscillator. The dependence of the frequency difference of the oscillator with the adsorption of water molecules on the number of monolayers in the film and the temporal change have been measured under a wide range of fixed relative humidity conditions. The humidity dependence of the sorption characteristics falls into two regions : the lower relative humidity region shows slow desorption/sorption and a linear dependence on the number of monolayers in the film, and the higher relative humidity region shows a rapid desorption/sorption process and is independent of the number of monolayers. In the lower relative humidity region, water sorption proceeds through the well ordered palmitoil cellulose LB monolayers onto various sites with different sorption energies in the glucose base unit. In the higher relative humidity region near saturation, the sorption is controlled simply by the free condensation of water molecules onto the him surface. These characteristics can be expressed numerically by the so-called Freundlich isotherm and the Polanyi theory. The applications of LB monolayer-based humidity sensors are briefly discussed.