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Abstract: Polymer nanoreactors incorporating gold nanoparticle catalysts were self-assembled via Flash Nanoprecipitation. The incorporated gold nanoparticles maintained catalytic activity, which was evaluated using reduction of 4-nitrophenol with sodium borohydride as a model reaction. The diffusion coefficient for 4-nitrophenol was determined by NMR and used to calculate the second Damk?hler number, which indicated that the systems were not diffusion limited. Despite similar diffusion coefficients, catalytic performance was strongly affected by the co-precipitant. For example, the apparent reaction rate per surface area using castor oil was over 8-fold greater than polystyrene. Thus, we measured the partition coefficient of 4-nitrophenol between water and castor oil or toluene (to mimic polystyrene). The core material:water partition coefficient for castor oil was 7.81 ± 0.16 compared to 0.09 ± 0.01 for toluene. Including the partition coefficient in the Langmuir-Hinshelwood model, the intrinsic rate constants were comparable. Overall, the increase in apparent catalytic performance could be attributed to differences in reactant solubility rather than differences in mass transfer or intrinsic kinetics. Finally, the polymer nanoreactor were applied to the one-pot condensation of benzaldehyde with 4-nitrophenol performed in water at ambient conditions. The product spontaneous precipitated from the reaction mixture; the nanoreactors were stable in dispersion whereas citrate-stabilized gold and PEG-coated gold precipitated with the product. The product (analytically pure by NMR) was extracted from the precipitate with acetone resulting in a yield of 66%. Overall, these results demonstrate proof-of-principle that multiple reactions can be performed in one-pot and the product can spontaneously phase separate from the nanoreactor dispersion.