We present the synthesis of molecularly controlled "CO2-switchable" polystyrene nanoparticles by surfactant-free miniemulsion polymerization using a carboxyl-functionalized surface-active monomer, which acts as comonomer and stabilizer at the same time. The obtained nanoparticles are about 100 nm in size and show a small size distribution, confirmed by dynamic light scattering (DLS) and electron microscopy. Under ambient conditions, the latex particles form a stable suspension that can be coagulated by bubbling CO2. The redispersion of the coagulated particles can be easily achieved by ultrasonication. The reversibility of the coagulation is confirmed after several coagulation/redispersion cycles (CO2 bubbling and ultrasonification) from DLS and zeta potential measurements.