The invention relates to a method for the preparation of trans- or cis-diammoniumdichlorodihydroxoplatinum(IV) and derivatives thereof. What is suggested is reacting trans- or cis-diammoniumdichloroplatinum(II) with a solution comprising >30% peroxide at temperatures below 30.degree. C. and dissolving the product thus obtained in a mineral acid and subsequently precipitating with an alkaline solution.