The growth and reduction of compact (alpha-) and hydrous (beta-) oxide films on polycrystalline Pt electrodes in aqueous 0.1 M NaOH solutions have been investigated using cyclic voltammetry, as well as in situ ellipsometry and the quartz crystal microbalance (QCMB) techniques. All cc-oxide films, formed in base with time at constant potentials up to 1.9 V, or by multi-cycling of the potential, are non-hydrated in nature, even when covered by a thick beta-oxide film. Two different forms of alpha-oxide film are generated, suggested to be PtO at lower potentials (less than or equal to 1.6 V) and PtO2 at higher potentials (> 1.6 V), based on their different growth rates and optical properties. All beta-oxide films are different from the alpha-oxide, being hydrated to varying extents. Based on the measured refractive index of 2.5, and their mass, thin beta-oxide films are suggested to be PtO2 .H2O, when formed using standard growth conditions (2 V/s between 0.5 and 2.8 V). As more beta-oxide film is formed, the outer regions become more hydrated (n as low as 1.8), and the suggested film composition is PtO(2 .)1.2H(2)O. Pt beta-oxide films are even more hydrated when formed using more negative E- and E+ limits during growth (while maintaining the other limit constant at either 0.5 or 2.82 V), resulting in film masses and a refractive index consistent with PtO(2 .)3.5H(2)O.