Use of nano-enabled products increases the potential for release of engineered nanoparticles (ENP) into the environment. Product weathering and further environmental transformations can create composite particles (CPs) that may contain multiple ENPs, a residual product matrix (e.g. polymer), or transformed/added surface coatings. Methodology that uses transmission electron microscopy (TEM), single particle ICP-MS (spICP-MS) and field-flow fractionation (FFF) was developed to facilitate the investigation of metallic ENPs associated with CPs. In this study, colloidal-sized nanoplastic CPs consisting of a gold–polymer nanocomposite (polystyrene-block-poly(acrylic acid)) were examined to reveal how combining FFF with spICP-MS can be used to characterize CPs. Metal NP size and particle number concentration is obtained by spICP-MS. Asymmetric flow field-flow fractionation (AF4) and centrifugal field-flow fractionation (CFFF) separate and size the CPs based on their hydrodynamic diameter and buoyant mass, respectively. Off-line spICP-MS analysis of fractions obtained by the FFF separations facilitated measurement of the mass and number of Au–PS (gold–polystyrene) ENPs (from 1 to >8) contained in the CPs. In particular, CFFF utilizing a carrier that was density-matched to the polymer proved very successful in measuring multiple gold ENPS in the CPs. The developed methodology can be applied to investigate ENP properties in environmental systems.