The electrochemistry of polycrystalline Pd has been examined in alkaline and acidic solutions, with the focus being on the growth and reduction of thin, hydrous Pd oxide (beta-oxide) films. In the first few cycles of potential between 0.47 and 2.8 V (reversible hydrogen electrode), the typical potential limits for beta-oxide growth employed in this work, a compact, anhydrous Pd oxide (alpha-oxide) film forms first. This alpha-oxide film has been suggested to consist of mainly PdO and some PdO2 at potentials positive of ca. 1.5 V. When the alpha-oxide film is ca. five monolayers in thickness, the conversion of the alpha- to beta-oxide commences at certain sites, resulting in the development of islands of alpha- and beta-Pd oxide. Evidence indicates that, with increasing oxide growth, the coverage of the electrode surface by patches of alpha-oxide of unchanged thickness decreases slowly, reaching a steady state, while the islands of beta-oxide lengthen in the form of columns.