Nanoparticles of copper(l) oxide (cuprous oxide; Cu2O) were able to be synthesized from nano-restricted copper acetate (Cu(OAc)(2)) in micropores of single-wall carbon nanotubes (SWNTs) by visible-light photoreduction. The specific structure of confined Cu(OAc)(2) in the micropore is indispensable for the reduction process to Cu2O by the irradiation, because, in general, aqueous solution of Cu(OAc)(2) can be reduced under UV-light irradiated conditions. The present results strongly suggest that the micropore of SWNTs whose pore width is in the micropore-size range can play as nanoreactor space for the synthesis of Cu2O through the nano-restricted precursor whose reactivity is different from that in the bulk phase. (c) 2014 Elsevier Inc. All rights reserved.