Stable oil-in-water Pickering emulsions (PEs) are prepared by using hydrophilic and oppositely charged spherical (silica) and nonspherical (hematite) nanoparticle mixture. By varying aqueous dispersion pH, the surface potentials of the particles are tuned. For fixed sizes of silica and hematite, stable emulsions are formed at low concentrations of silica when the surface potential mismatch between the particles is high. Conversely, when the latter is small, stable emulsions are found up to high concentration of silica nanoparticles. These findings are explained based on the formation of silica-hematite heteroaggregates and on their surface-active characteristics. It is found that for a given pH and particle concentration (expressed as number ratio NS-H, which is the ratio of the total number of spheres to the total number of spheroids), highly stable oil-in-water emulsions are formed only when the unstable aqueous binary mixture of colloids and the oil phase are emulsified. The result is presented as a state diagram, which shows the particle composition corresponding to the formation of stable emulsions, the composition range where the emulsions that result are unstable and the region where the emulsions do not form at all. As the formation of PE is kinetically controlled, we demonstrate that at conditions where emulsions formed via one-step emulsification for a given NS-H are unstable, following a two-step emulsification procedure yields highly stable emulsions. The work highlights the role of (i) NS-H, (ii) aqueous dispersion pH, (iii) surface potential mismatch and (iv) emulsification protocol on the formation and stability of solid particle stabilized emulsions. © 2020 Taylor & Francis Group, LLC.