The transformations of Pt complex species in concentrated NaOH solutions (1-12 M) of Na2[PtCl6] were studied with a combination of methods, including 195Pt nuclear magnetic resonance, ultraviolet-visible, and Raman spectroscopy. The two-step process was observed under the following conditions: (1) formation of the [Pt(OH)5Cl]2- anion that proceeds relatively fast even at room temperature and (2) further slow substitution of the last chlorido ligand with the formation of the [Pt(OH)6]2- anion. Overall, it was determined that the [PtCl6]2- to [Pt(OH)6]2- transformation (especially the first stage) is greatly accelerated under blue light (455 nm) irradiation. The structures of [Pt(OH)Cl5]2- and [Pt(OH)5Cl]2- were determined using the single-crystal X-ray diffraction data of the corresponding salts isolated for the first time. Analysis of the [Pt(OH)Cl5]2- reactivity showed that under analogous conditions, its hydrolysis proceeds 2 orders of magnitude slower than that of [PtCl6]2-, indicating that the formation of [Pt(OH)5Cl]2- from [PtCl6]2- (stage 1) does not follow a simple sequential substitution pattern. A model for [Pt(OH)5Cl]2- anion formation that includes the competing reaction of direct Cl ligand substitution and the self-catalyzed second-order reaction caused by a redox process is proposed. The influence of Pt speciation in alkaline solutions on the reductive behavior is shown, illustrating its impact on the preparation of Pt nanoparticles.