Voice disguise is potentially present in any speech-specific criminal case. Appropriate methods of analysing and evaluating voice disguise therefore are necessary. An experiment was conducted in which 20 subjects provided samples of their modal as well as disguised voices. T he goals were to determine the preferred forms of disguise and possible relations between the modal voice properties and the chosen alterations. It was found that the majority of the disguises were made by an alteration of phonation. Also, the disguises were based on an alteration of a maximum of two phonetic parameters, leaving broad aspects of the vocal behaviour undisguised and thus available for forensic examination.