The dielectric properties of aluminium oxide (Al2O3) thin films obtained by plasma-enhanced atomic layer deposition (PEALD) from Al(CH3)3/O2 precursors were investigated while focusing on the influences of thermal annealing under a dioxygen (O2) ambient. PEALD-Al2O3-based metalinsulator-silicon structures/capacitors and pH-sensitive chemical field effect transistors were fabricated to deal respectively with microelectronic applications and measurement in liquid phase. Antagonist results were thus evidenced. On the one hand, dealing with high-k gate materials, optimized dielectric properties, i.e. low fixed charge density (4 × 10 12 cm-2), high dielectric constant (r = 10.2), Fowler-Nordheim conduction and high breakdown electric fields (Ebd = 8.75 MV/cm) were obtained for polycrystalline PEALD Al2O3 films annealed at high temperature (T > 750°C). On the other hand, pH detection properties, i.e. quasi-Nernstian sensitivity (59 mV/pH), low drift (< 1 mV/day) and long lifetime (> 180 days), were optimized for unannealed amorphous PEALD Al2O3 films. These phenomena were associated germination/crystallization phenomena in the deposited amorphous alumina structure as well as to related charge trapping and leakage currents in water-based solutions related to the final Al2O3 polycrystalline structure.