This essay analyses optimal voting rules for one form of deliberative democracy. Drawing on public choice analysis, it is argued that (i) the voting rule that best institutionalises deliberative democracy is a type of a supermajority rule. Deliberative democracy is also committed to (ii) the standard neutrality condition according to which if x votes are enough to select alternative A, x votes must be enough to select not-A. Taken together, these imply that deliberative democracy will often be indeterminate. This result shows that deliberative democracy is ill-equipped to provide guidance as to how actual political disputes are to be legitimately resolved.