To better understand the interactions between nanoparticulated whey protein (NWP) and other milk proteins during acidification, milk model systems were diluted to 0.5% protein concentration and adjusted to pH of 6.0-4.5 following homogenisation and heat treatment. The diluted systems with different concentrations of NWP (0-0.5%) were characterised in terms of particle size, viscosity, surface charge and hydrophobicity. When pH was adjusted to 5.5, aggregation was initiated at levels of NWP (0.25-0.5%) leading to significant increase in particle size and viscosity. Pure NWP (0.5%) showed largest initial surface charge (-27 mv) and higher surface hydrophobicity than the other systems. The results indicated that NWP could self-associate above pH 5.5 and not only the decrease of electrostatic repulsion but also other interactions, such as hydrophobic interaction, play an important role in contributing to the early self-association of NWP.