Moisture sorption, glass transition temperature (T-g) and morphology of ultra thin poly(chloro-p-xylylene) (parylene-C) are greatly influenced by geometrical confinement effects. For film < 50nm, the equilibrium moisture saturation is a decreasing function of film thickness. However, the T-g of film < 50 nm is about 10 degrees C higher than thicker films. The above phenomena are attributed to the effect of geometrical confinement on the thermal properties and the morphology of parylene-C film. Surface confinement results in an increased in Tg, but a decreased in crystallinity for films < 50 nm. In this study, we show that the increase in moisture sorption for parylene-C films < 50 nm is dominated by the crystallinity variation. (c) 2005 Elsevier Ltd. All rights reserved.