Parylenes are used for a wide range of applications in microelectromechanical systems (MEMS) devices. However, their poor adhesion in a harsh liquid environment can limit the fabrication processes of complex MEMS and bioMEMS devices. Potassium hydroxide (KOH) wet etching is particularly challenging and was used to evaluate the adhesion of Parylene C on silicon, silicon nitride, and silicon dioxide substrates. Using a number of characterization procedures, this paper shows that the delamination is the result of liquid penetrating both at the Parylene-substrate interface and through the polymer layer. The combination of an adhesion promoter and a thermal treatment improves the adhesion of the layer. The treatment is evaluated in two case studies, where the Parylene is used as: 1) a biocompatible coating, and 2) as a mask to block the entrance of a microfluidics channel. In the first case, it is shown that the treatments, including the KOH exposure, do not influence the growth and proliferation of SaOS-2 cells, as compared to a generic Parylene layer. In the second case, the results show that Parylene can be used efficiently to block the entrance of the channel, and that it can be removed afterward.