In this study, we explore the role of extractant and diluent molecular structure on organic phase aggregation in binary mixtures of extractant and diluent. (Here, we define compositions by the extractant volume fraction, φ E .) We consider linear alkane diluents with a family of malonamide extractants, shown in Figure , which are commonly applied to lanthanide separations. We use small angle x-ray scattering (SAXS) to quantify the concentration fluctuations associated with the phase instability over the full range of extractant/diluent compositions, for diluents and extractants of various molecular lengths. For the organic phase having a critical temperature, T C , closest to room temperature, we measure temperature-dependent SAXS patterns to obtain critical exponents. Then, we investigate this mechanism of organic phase aggregation with molecular dynamics (MD) simulations.