With the increasingly widespread use of titanium dioxide nanoparticles (TiO2NPs), the particles' environmental impacts have attracted concern, making it necessary to understand the fate and transport of TiO2NPs in aqueous media. In this study, we investigated TiO2NP aggregation caused by the effects of humic acid (HA), ionic strength (IS) and different pH using dynamic light scattering (DLS) to monitor the size distribution of the TiO2NPs continuously. It was determined that HA can influence the stability of TiO2NPs through charge neutralization, steric hindrance and bridging effects. In the absence of IS, aggregation was promoted by adding HA only when the pH (pH=4) is less than the point of zero charge for the TiO2NPs (pHPZC≈6) because HA reduces the zeta potential of the TiO2NPs via charge neutralization. At pH=4 and when the concentration of HA is 94.5μg/L, the zeta potential of TiO2NPs is close to zero, and they reach an aggregation maximum. A higher concentration of HA results in more negatively charged TiO2NP surfaces, which hinder their aggregation. When the pH is 5.8, HA enhances the negative zeta potential of the TiO2NPs and increases their stability via electrostatic repulsion and steric hindrance. When the pH (pH=8) is greater than pHpzc, the zeta potential of the TiO2NPs is high (~40mV), and it barely changes with increasing HA concentration. Thus, the TiO2NPs are notably stable, and their size does not grow at pH8. The increase in the critical coagulation concentration (CCC) of TiO2NPs indicated that there is steric hindrance after the addition of HA. HA can enhance the coagulation of TiO2NPs, primarily due to bridging effect. These findings are useful in understanding the size change of TiO2NPs, as well as the removal of TiO2NPs and HA from aqueous media. © 2014 Elsevier B.V.