Hetero-aggregation refers to aggregation of particles that are not identical i.e. particles of different physical-chemical properties. The investigation of this phenomenon is important because of the fascinating structures that can be formed and their application in several fields including the synthesis of porous materials and particle stabilized emulsions. We report an experimental study of hetero-aggregation behaviour of oppositely charged nanoparticles of similar size. In this study, the hetero-aggregation phenomenon is investigated using a combination of visual observation, zeta potential measurements, dynamic light scattering, scanning electron microscopy and rheology measurements. We report details of aggregate size, structure, flow properties to provide understanding of hetero-aggregation by a careful examination of different phases formed upon mixing oppositely charged particles. The experiments were carried out at different mixingfraction (defined as the mass of positive particle in the dispersion divided by total mass of particles in the dispersion) varying from 0 to 1 with total concentration of particles ranging from 0.05 to 30 wt% (0.023-13.82 vol%). At low total particle concentration, four different states of the mixture were observed which includes sediment with turbid supernatant, sediment with clear supernatant, turbid sample with no sediment and a clear dispersion. However, at higher concentration above similar to 7.5 wt% (3.45 vol%), the mixture of oppositely charged particles form - a particulate gel with turbid supernatant at low mixing fraction (from 0.1 to 0.3), a solid-like gel at intermediate mixing fraction (from 0.3 to 0.7) and a turbid sample at high mixing fractions from 0.7 to 1.0. (C) 2016 Elsevier Inc. All rights reserved.