The application of the ALD process to continuously moving flexible substrates required for a true roll-to-roll process brings with it a number of issues. Some are similar to those in batch ALD but others are new problems due to the need to ensure the separation of precursor gas streams in space rather than in time. In ASTRaL, we have developed a continuous ALD (CALD) system, in conjunction with Beneq Oy, as a tool to explore these issues and act as a test bed for the development of a true roll-to-roll process1. This is a system where the substrate, mounted on a rotating drum, moves through the precursor and purge zones sequentially (fig. 1). In the CALD system, using the TMA/water process for the deposition of aluminium oxide, the general behaviour is similar to that of a conventional ALD process. That is, for low speeds of substrate movement, equivalent to high precursor exposure, the deposition rate saturates whereas when the speed is increased above a threshold value, then the deposition rate decreases due to incomplete saturation of the substrate surface with precursors. However, under certain conditions, anomalous growth behaviour takes place which is caused by a combination of gas entrainment by the moving substrate, incomplete purging as it moves through the purging zone and the slow desorption of excess water on the surface. This gives increased growth rate at certain speeds which is a function of the process temperature. To get a better understanding of the genesis of these effects, we have measured the effect of gas leakage between the various precursor and purge zones as a function of flow rates and substrate speed and linked these to variations in the gas boundary layer and the purging efficiency.