<p>The current invention relates to new HPLC methods for the analysis of the drug substance vorinostat and related substances. In a first method the mobile phase comprises two or more liquids and the relative concentration of the liquids is varied to a predetermined gradient. In a second method the mobile phase comprises an alcohol. In a third method the mobile phase comprises a phosphate salt, a phosphoric acid or an anhydride thereof. The current invention also relates to a method for analysing a substance, comprising the detection and optional quantification of one or more specific impurities. The current invention further relates to the use of specific substances as reference standards and markers, and to vorinostat and associated pharmaceutical compositions that have been analysed by the methods of the current invention and/or are substantially free of specific impurities.</p>
