Political parties respond to electoral rules in ways which gain them partisan advantage and enable them to make strategic choices about the use of their electoral support. The alternative vote (AV) and proportional representation by the single transferable vote (STV) provide considerable opportunity for this kind of partisan activity. The ability of the voter under such electoral systems to rank candidates in order of the voter's preference creates a kind of property which can be used by parties, especially minor parties, to influence the behaviour of both candidates and other parties. The paper investigates this aspect of preferential voting systems and the extent to which the context of electoral rules can encourage or discourage a trade in partisan preferences. Elections for the Australian House of Representatives and Senate are used to show how political actors can respond to electoral rules which permit the control and trading of preferences to be developed into a series of sophisticated transactions. (C) 2002 Elsevier Science Ltd. All rights reserved.