In this study, the zeta potential and z-average hydrodynamic diameter of zinc oxide (20 nm, 99.5% purity) were investigated as a function of pH and time. The nanoparticles readily formed aggregates and could not be disintegrated to its primary size eventhough ultrasonication was applied. In zinc oxide suspensions (25 mgL and 100 mg/L) the nanoparticles were examined for electrophoretic mobility and aggregation behaviour within the pH range of 3 to 11 and time duration of 60 minutes. In this study, the particle size and zeta potential were measured by dynamic light scattering technique. The application of nanoparticle tracking analysis was demonstrated as well to observe the aggregation and disaggregation of zinc oxide nanoparticles at various pH values. The NTA results obtained in this work were compared and validated by the DLS. The z-average hydrodynamic diameter of the particles was 389 d.nm with an average zeta potential value of +23 mV using the DLS compared to mean particle size of 365 d.nm by NTA. DLS analysis also identified the pH of the point of zero charge (pHPZC) and the isoelectric point (IEP) at pH 9.4 and pH 6.4 respectively. It was found that the zeta potential remained unaffected by the concentration of the suspension. However, results show that pH does have a pronounced effect on the stability of zeta potential, pH and particle size as a function of time in pH regions where dissolution (< pH 6.4 and 8.3 < pH < 10.5) and aggregation (pH 6.4 and 9.4) occurs. It is apparent that the pH of the suspension strongly influences the zeta potential of ZnO nanoparticles without external factors which will consequently govern their behaviour, agglomeration state and their interactions with each other and surfaces in the environment.