The acid-base properties of alumina (α-Al2O3) of different particle size classes are studied at room temperature by potentiometric titrations. The obtained results show that the point of zero charge (PZC) is unstable at t ≤75 min for grain size classes below 32-63 μm and varies between 3.3 ± 0.2 and 6.5 ± 0.2 for the classes 63-200 μm and 42-27 μm. The PZC is obtained respectively up to 3.1 ± 0.1 and 3.6 ± 0.1. The isoelectric point (IEP) which is the intersection of the suspensions titrations curves at different contact times was obtained at 9.5 ± 0.1. However, the IEP is difficult to observe when the particle size is less than 23-63 μm.