Infrared spectroscopy was performed on a thin section of a chalcedonic quartz at high temperature in order to investigate the states and diffusivity of intergranular water. The sample contained 0.3 wt.% of silanol (Si-OH) and 0.3 wt.% of molecular H2O, located mainly at intergranular regions but also as fluid inclusions. We monitored the diffusion of molecular H2O associated with dehydration by in situ analyses at 350-500°C and determined the bulk-diffusion coefficients as expressed by an Arrhenius relationship: Dbulk (m2 sec -1) = 10 -4.5 exp(-107±17/RT), where R is the gas coefficient and T the temperature. The activation energy for our sample is similar to those previously reported for diffusion in quartz aggregates with incompletely-connected grain boundaries. This result and our previous measurements of electrical conductivity imply that diffusion of molecular H2O at incompletely-connected intergranular regions is the main mechanism for the dehydration. The diffusion coefficients in chalcedony are larger than those previously reported for rhyolitic glass and other granular aggregates. Intergranular regions, inherent larger pores and cracks created during heating can act as efficient diffusion paths within the chalcedony.