We report a new synthesis of colloidal gold(I) sulfide nanoparticles by reacting aqueous solutions of sodium sulfide and sodium gold sulfite. Optical absorption spectra and pH were measured during the reaction. The pH changes from 10.3 to 8.8 immediately after mixing the reactants and very slowly decreases during particle growth. The nanoparticles exhibit a continuous increase in absorption as the photon energy increases from 1 to 5 eV. The optical band gap of the nanoparticles has been estimated from the onset of absorption to be 1.8 +/- 0.2 eV. X-ray photoelectron and energy-dispersive X-ray spectroscopy indicate that the composition of the particles is Au2S. The measured S(2P(3/2)) and Au(4f(7/2)) binding energies suggest that Au2S has mixed covalent/ionic bonding. Transmission electron microscopy shows that the particles are roughly spherical with an average diameter of 4 nm.