Janus nanoparticles with a poly(l-lactide) face and a polystyrene-based face functionalized with amine or carboxylic acid groups were synthesized via two different approaches. In the first approach, the poly(styrene-co-methacrylic acid) or poly(styrene-co-2-aminoethyl methacrylate) copolymers were generated in situ in miniemulsion droplets before phase separation between the copolymers and the poly(l-lactide) occurred. In the second approach, the copolymers were prepared before the emulsification step. A solution containing the poly(l-lactide) and one of the copolymers was then emulsified, and the solvent was subsequently removed to induce a phase separation between the polymers, yielding a Janus morphology. The density of functional groups (amine or carboxylic acid) could be varied between 0 and 5 groups per nm(2). Finally, we demonstrated that one face of the Janus nanoparticle could be selectively employed for a chemical reaction. Indeed, silver nanoparticles could be nucleated selectively on the poly(l-lactide) face.