Water chemistry affects the nucleation kinetics, precipitate morphology, and quantity of ironIJIII) (hydr)oxide nanoparticles, directly impacting the reactive surface area of geomedia and fate of associated waterborne contaminants. In this study, we utilized in situ grazing-incidence small angle X-ray scattering (GISAXS) and complementary ex situ techniques to investigate heterogeneous ironIJIII) (hydr)oxide nucleation on quartz in the presence of natural organic matter (NOM) and arsenate. Results indicate unique fractal aggregation behavior in the systems containing NOM and precipitating ironIJIII) (hydr)oxide nanoparticles. Furthermore, the coexistence of arsenic and NOM lead to the formation of two distinct particle size ranges: larger particles dominated by arsenic effects, and smaller particles dominated by NOM effects. These new findings provide important implications for understanding the nucleation, growth, and aggregation of ironIJIII) (hydr)oxides in aqueous systems where NOM is present, such as natural surface waters and water and wastewater treatment plants. This study also offers new insight into how NOM-associated ironIJIII) (hydr)oxides can interact with aqueous contaminants such as arsenate.A better understanding of the fate and transport of ironIJIII) (hydr)oxide nanoparticles is crucial for controlling trace toxic contaminants in natural and engineered aquatic systems. Despite this, their nucleation, growth, and aggregation are not well understood. We studied the impacts of natural organic matter and arsenate on newly-forming ironIJIII) (hydr)oxide in situ and found that these constituents altered the nanoparticle size and aggregation state. Our findings allow us to elucidate how contaminants affect the aggregation or disaggregation of ironIJIII) (hydr)oxide nanoparticles in aqueous environments, where organic matter will be nearly ubiquitously present. Outcomes can also be informative in developing reactive transport models that better predict the ability of ironIJIII) (hydr)oxides to attenuate aqueous contaminants.