This Study describes an original approach to prepare biphasic particles by simultaneous ring-opening metathesis polymerization (ROMP) of norbornene and atom-transfer radical polymerization (ATRP) of methyl methacrylate under miniemulsion conditions. A single water-soluble ruthenium macrocatalyst was used to initiate ROMP and mediate ATRP. All situations led to high conversions, and well-distributed particles containing two incompatible homopolymers grown under controlled conditions could indeed be prepared.