The directed or dynamic assembly of molecular components in solution is a simple and effective strategy to confine materials in desired geometries and length scales. We use a kinetic control strategy with block copolymer blending to construct complex nanoparticles through the demixing of unlike block copolymers within the same nanoscale particle. Successful nanoparticle construction relies on kinetic trapping of unlike block copolymers into the same nanoparticle with solution processing. Not only can we make nanoparticles with multiple internal compartments of a desired size, but we can also make nanoparticles of hybrid geometries (e.g. a blend of cylindrical and spherical geometries). These combination particles are kinetically trapped, non-equilibrium structures. However, the block copolymers are able to phase separate locally within the nanoscale particle, thus producing internal compartments and hybrid geometries.