Interfacial space charges significantly influence transport and recombination of charge carriers in optoelectronic devices. Due to the mixed ionic-electronic conducting properties of halide perovskites, not only electronic effects, but also ionic interactions at their interfaces need to be considered in the analysis of space charges. Understanding of these interactions and their control is currently missing. This study elucidates the ionic effects on space charge formation at the interface between methylammonium lead iodide (MAPI) and alumina, and its modulation through surface modification using organic molecules. Embedding insulating alumina nanoparticles within MAPI films leads to enhancement of the electronic conductivity. This effect is consistent with the formation of an interfacial inversion layer in MAPI and can only be explained on the basis of ionic interactions. Such an effect is attenuated by surface modification of the oxide via the chemisorption of organic molecules. Finally, the same trend is observed in solar cells, where reducing the potential of the distributed space charges within the composite active layer improves device performance. These findings emphasize the necessity of taking into account ionic interactions to control the space charge formation at interfaces involving mixed ionic-electronic conductors, an essential aspect in the performance optimization of halide perovskite-based devices.|The space charge equilibrium at MAPI/alumina interfaces is dictated by ionic equilibration. Ionic space charges are modulated via surface-adsorbed organic molecules. These hinder ionic adsorption from MAPI to the contact phase, reducing the interfacial space charge potential and the concentration of accumulated electrons. Suppressing the formation of distributed space charges in the active layer of solar cells improves device performance. image