Aqueous solutions of Pt(II) (aged PtCl42- solutions) are reduced by hydrogen in the absence of a stabilizer and in the presence of stabilizing sodium citrate or/and sodium hydroxide. The rate of reduction is measured optically and conductometrically, and the colloidal nanoparticles, which are finally formed, are studied by high-resolution electron microscopy. In dilute solutions, the rate of reduction is proportional to the square of the Pt(II) concentration. In the presence of citrate and NaOH, the reduction occurs more slowly. A mechanism, in which intermediate free Pt atoms do not appear, is discussed. Monovalent platinum is postulated as the first intermediate, and an autocatalytic contribution to the reduction is observed. Particle growth in a chain reaction is also discussed. In the presence of NaOH, the reaction occurs in two steps: formation of a hydroxilated precursor and reduction of the latter. Depending on the concentration of the stabilizing additives, colloids consisting of polyhedra of different shape (cubic, tetrahedral, and triculent octahedral) are formed. Stabilization by citrate plus NaOH yields particularly long lifetimes of the colloidal solutions.