The synthesis, characterization, and proper,ies of colloidal nanostructures in hydrocarbon media are reported. These sterically stabilized structures are unusual in that the stabilizing arms are attached to the particle by photoactive groups, and the arms can be cleaved through irradiation, leading to photoinduced flocculation. In the first step of the synthesis, cross-linked poly(butyl methacrylate) (PBMA) latex particles with controlled functionality (-COOH and -OH groups) were prepared by emulsion polymerization. The purified PBMA latex was neutralized with Cs2CO3 and then transferred to an organic medium (dimethylformamide), where it was converted to lipophilic nanoparticles through chemical modification with 2-bromo derivatives of long-chain 1-phenylalkyl ketones or nitrobenzyl ether derivatives of polyisobutylene, The particles can be dispersed in hydrocarbon solvents. Irradiation leads to flocculation, except that in the presence of hydrocarbon-soluble polymers bearing amino groups, the unprotected particles can be intercepted and flocculation inhibited. These particles serve as an interesting model for precursors to sludge formation in motor oil.