The heteroaggregation of engineered nanoparticles (ENPs) with natural colloids (NCs), which are ubiquitous in natural surface waters, is a crucial process affecting the environmental transport and fate of ENPs. Attachment efficiencies for heteroaggregation, α_hetero, are required as input parameters in environmental fate models to predict ENP concentrations and contribute to ENP risk assessment. Here, we present a novel method for determining α_hetero values by using a combination of laser diffraction measurements and aggregation modeling based on the Smoluchowski equation. Titanium dioxide nanoparticles (TiO₂ NPs, 15 nm) were used to demonstrate this new approach together with larger silicon dioxide particles (SiO₂, 0.5 μm) representing NCs. Heteroaggregation experiments were performed at different environmentally relevant solution conditions. At pH 5 the TiO₂ NPs and the SiO₂ particles are of opposite charge, resulting in α_hetero values close to 1. At pH 8, where all particles are negatively charged, α_hetero was strongly affected by the solution conditions, with α_hetero ranging from <0.001 at low ionic strength to 1 at conditions with high NaCl or CaCl₂ concentrations. The presence of humic acid stabilized the system against heteroaggregation.