When a silicone sealant is exposed to wet air, a surface skin is formed, which becomes thicker with time. For a 1-part room temperature vulcanization (1-RTV), neutral cure system based on the hydrolysis and condensation reactions of alkoxysilanes, it has been observed that, at constant temperature and humidity, the thickness of the cured layer is initially proportional to the square root of time, but later the gradient of such a plot increases. There are thus two regions of cure, an outer region and an inner one. Swelling in toluene, which has been measured for samples taken at various depths, shows that the cross link density is greater in the outer regions. Single lap joints of aluminium or glass have been cured in these two regions and joints from the outer region are always stronger than the corresponding ones from the inner region. This behaviour has been ascribed to the mobility of low molar mass cross linking and coupling agents. During the initial stages of cure they migrate into the outer region, but once cure has passed into the inner region there is now a paucity of these compounds. Young's modulus and the concentration profile of silane crosslinking agent have also been measured. All observations confirm migration of crosslinking species, and gradient formation for most physical properties in the final joint. Measurement of the water vapour permeability coefficient (P) of the cured siloxane network has shown that P is independent of humidity and stretching of the material. A plot of log P against reciprocal absolute temperature is non-linear with positive slope, showing that the heat of permeation is negative and varies with temperature. This can be explained by the formation of water-clusters. When the network is formed water is absorbed and alcohol is released.