Irradiation studies of acetonitrile solutions of copper(II) acetate dimer ([Cu(OAc)2]2) using high energy, simultaneously spatially and temporally focused (SSTF) ultrashort laser pulses are reported. Under ambient conditions, irradiation for relatively short periods of time (10-20 s) selectively produces relatively small, narrowly size-dispersed (3.5 ± 0.7 nm) copper(I) oxide nanoparticles (Cu2O NPs) embedded in CuCN-polyacetonitrile polymers generated in situ by the laser. The Cu2O NPs become embedded in a CuCN-polyacetonitrile network as they form, stabilizing them and protecting the air-sensitive material from oxygen. Laser irradiation of acetonitrile causes fragmentation into transient radicals that initiate and terminate polymerization of acetonitrile. Control and mechanistic investigations reveal that HCN formed during laser irradiation reacts rapidly to reduce the Cu(II) centers in [Cu(OAc)2]2, leading to the formation of CuCN or, in the presence of water, Cu2O nanoparticles that bind and cross-link CuCN-polyacetonitrile chains. The acetate-bridged Cu(II) dimer unit is a required structural feature that functions to preorganize and direct the Cu(II) reduction and selective formation of CuCN and Cu2O nanoparticles. This study illustrates how rapid deposition of energy using shaped, ultrashort laser pulses can initiate multiple photolytic and thermal processes that lead to the selective formation of composite nanoparticle/polymer materials for applications in electronics and catalysis.