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The characterization of nanoparticles (NPs) in hydrocarbon matrices using single particle inductively coupled plasma mass spectrometry (spICP-MS) is underdeveloped. There are less than ten publications using spICP-MS in hydrocarbon matrices, and none have applied the technique to determine NP concentration and size distribution in asphaltenes after in-situ upgrading of heavy oils via solvent deasphalting. To our knowledge, no studies have used spICP-MS to track the nature of NP additives in the asphaltene fraction in hydrocarbons without adulteration of the sample. Particle number concentrations (PNC) derived from spICP-MS in hydrocarbon matrices are reported for the first time. Fe2O3 PNC increased by an order of magnitude, and NiO PNC increased 28 % compared to samples without additives, indicating that NPs were reasonably well-dispersed in the asphaltenes. Ionic concentrations were higher for Ni than Fe, which showed negligible changes in all samples. Here, we report the lowest size detection limits recorded for Fe2O3 NPs (32 nm ± 1 nm) using spICP-MS in hydrocarbon matrices. Further, NiO and Fe2O3 NP sizes matched the initial sizes added to the oil before precipitation, providing evidence that the nature of the NPs does not change after deasphaltation and subsequent mixing with asphaltenes. This study expands our understanding of the interactions between metal NPs and asphaltenes when used as co-precipitants during in situ upgrading of heavy crude oil.