The radiolytic reduction of PtCl42- in the presence of 10(-3) M sodium polyphosphate (2 x 10(-4) to 5 x 10(-3) M) and 10(-2) M methanol yields colloidal Pt particles of irregular shape, 10-30 Angstrom in diameter ("radiolysis" colloid). The reduction of PtCl42- is partly autocatalytic. The reduction of the aquated complexes, (PtCl2(H2O)(2) and PtCl3(H2O)(-), by hydrogen in the presence of 5 x 10(-4) M sodium polyacrylate or polyphosphate leads to colloidal particles of regular shape, 50-80 Angstrom in diameter ("hydrogen" colloid). The absorption spectrum of the "radiolysis" and "hydrogen" colloids is compared to the previously reported spectra of colloids which were prepared using citrate or poly(vinyl alcohol) as stabilizer. The spectra of the presently prepared colloids have the predicted absorption maximum at 215 nm. The Pt particles tend to form clusters, whose absorption spectrum is rather flat with a maximum at 260 nm. A few reactions of the new colloids are described and their reactivities compared to that of a colloid prepared by citrate reduction of PtCl62-. Significant differences in the reactivity toward O-2, H-2, and Au(CN)(2)(-) exist between the various colloids as well as in their ability to catalyze the autoxidation of propanol-2. The differences are discussed in terms of differences in particle size and in the oxidation state of the particle surface.