Compound-semiconductor-substrate polishing methods, compound semiconductor substrates, compound-semiconductor-epitaxial-substrate manufacturing methods, and compound semiconductor epitaxial substrates whereby oxygen superficially present on the substrates reduced. A compound semiconductor-substrate polishing method includes a preparation step (S10), a first polishing step (S20), and a second polishing step (S30). In the preparation step (S10), a compound semiconductor substrate is prepared. In the first polishing step (S20), the compound semiconductor substrate is polished with a chloric polishing agent. In the second polishing step (S30), subsequent to the first polishing step (S20), a polishing operation utilizing an alkaline aqueous solution containing an inorganic builder and having pH of 8.5 to 13.0 inclusive is performed.