Tetrabutylammonium hydroxide/water mixtures are efficient reaction media for the fabrication of nanoscale metal oxides and hydroxides. Uniform CuO nanoplates, among others, can be grown on a large scale. This work shows that after 30 s at temperatures above 40 degrees C, CuO formation is already essentially complete. X-ray diffraction and transmission electron microscopy (TEM) show that the resulting plates form via a two-step process, where Cu(OH)(2) rods precipitate first. These rods aggregate and fuse into plates with a width/height ratio of about 1.9. High-resolution TEM and electron diffraction show that the plates are single crystals and exhibit only some defects, which most likely originate from the assembly and fusion of the primary rods.