Hypothesis: To study molecular exchange between colloids requires the preparation of suitably labelled species. Deuterium isotopic labelling has been used to prepare two chemically identical yet isotopically distinguishable poly(lauryl methacrylate)-poly(methyl methacrylate) (PLMA-PMMA) diblock copolymer colloids by polymerisation-induced self-assembly (PISA) directly in an alkane solvent. Molecular exchange should be detectable by performing small-angle neutron scattering (SANS) measurements on the dispersions. Experiments: SANS measurements were performed on fully hydrogenous PLMA(39)-PMMA(57) and deuterated core PLMA(39)-P(MMA-d(8))(57) colloids. They were mixed in equal amounts and heated to determine if molecular exchange occurred. PISA syntheses are often thermally initiated, and diblock copolymers self-assemble at elevated temperature, making this an important parameter to study. Experimental data for the mixture were compared to predicted curves for exchanged and non-exchanged colloids. Findings: The scattering of a mixture of fully hydrogenous and deuterated core copolymers does not disappear upon molecular exchange, due to the remaining contrast between the stabiliser and the core and solvent even after the cores fully exchange. By simultaneously fitting the SANS data from dispersions before mixing and using these parameters to constrain fitting the SANS data of mixtures, the molecular exchange between diblock copolymer micelles upon heating is clearly observed. (C) 2020 The Author(s). Published by Elsevier Inc.