Abstract: Directly elected mayors in England are currently elected by a voting system known as the Supplementary Vote (SV). This method was first used to elect the London mayor in 2000 and used also for all other subsequent mayoral elections. Only when voters use either their first or their second vote (or both) for one of the top two candidates, can their vote be called 'effective'. If they do not vote for one of the top two candidates, their vote is wasted and ineffective in deciding between the top two candidates. If the group of ineffective vote(r)s is large, the outcome might not reflect the preferences of the electorate. By identifying seven types of voters within SV, we show which groups of voters may be ineffective in expressing their preference between the top two candidates. We discuss the way voters have or have not effectively used SV in London mayoral elections in both 2000 and 2004. At the London mayoral elections of 2000 and 2004, about 20 per cent of the voters were, in this respect, ineffective. This high percentage resulted in the election of a mayor with less than a clear majority of the votes.