Water-dispersible, isolable thiolate-stabilized platinum nanoparticles are synthesized by two one-phase methods: one is conducted in water; another mainly in methanol, through the reduction of chloroplatinic acid with sodium borohydride, using mercaptosuccinic acid (MSA) as the stabilizer. The well-separated particles with mean diameter of 2.5 nm are obtained in water when the initial molar ratio of MSA to chloroplatinic acid (S/Pt) is 0.7. High-resolution transmission electron microscopy reveals that most particles are fee single crystals of cuboctahedral or truncated octahedral morphology with cell parameter similar to that of bulk Pt. The average particle diameter increases from 2.5 nm to around 4.7 nm upon decreasing the S/Pt ratio from 0.7 to 0.1; at the same time; particle aggregation occurs due to the incomplete coverage of thiolate molecules on the particle surface. The particles with mean diameter of 2.3 nm are also obtained while using methanol as the solvent at an initial S/Pt ratio of 0.5. Experiments through changing the concentration of sodium borohydride show that the reaction is thermodynamically controlled; the final Pt particle size is dictated by the initial S/Pt ratio.