Reactive impingement mixing was employed to produce polymer-protected nanoparticles. Amphiphilic block copolymer was formed in situ by reactive coupling of hydrophobic and hydrophilic blocks. Simultaneously, a hydrophobic compound and the copolymer coprecipitated to form nanoparticles in the range of 100 nm. Specifically, β-carotene was stabilized by the amphiphilic diblock copolymer, formed from the reaction of an amino-terminated hydrophilic block, polyethylene glycol) (PEG-NH 2), with an acid chloride-terminated hydrophobic block, either poly (ε-caprolactone) (PCL-COCl) or polystyrene (PS-COCl). Spherical particles were observed by scanning and cryogenic transmission electron microscopy. Process conditions, including feed concentration of β-carotene and feed concentrations of polymeric stabilizers, had little or no effect on average particle sizes over the range studied. Further, for Reynolds numbers greater than 500 the feed flow rates also had no effect. The effect of glass transition temperature (T g) of the hydrophobic polymer on morphology and particle formation mechanism is discussed.