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The main difference between dry and immersion lithography lies in the interactions between the immersion medium and photoresist. For example, it has been shown that resists components as PAG or base additive can leach into water. Such leaching could degrade the resist performances and the optical elements such as scanner lens. Moreover, it could be involved in new defectivity mechanisms as watermark formation. That's why immersion dedicated resists with low leaching rate or top coat protective layers have been designed for immersion lithography. However, they do not permit to prevent all leaching and there is always some residual amount of PAG extracted through the barrier layer. Even if leaching phenomena is now well understood for conventional resists, some questions are still present concerning PAG leaching through top coats. In this paper, we present a study of PAG leaching mechanisms through top coats. It is based on leaching kinetics analysis by HPLC/MS. As described in the literature, PAG leaching from dry resists into a static water volume has been shown to follow a first order kinetics model, however, in the case of a top coat stack, we observed a very different behaviour. A new model, taking into account both resist and top coat surface leaching has been developed to fit the experimental data. It illustrates the presence of two leaching mechanisms: a fast one from the top coat surface and a slow one from an embedded interface. This model also provides interesting data about the leaching process on immersion dedicated resists.