AbstractApplying a framework of perfect competition under uncertainty, we contribute to the discussion of whether or not ad valorem taxes and specific taxes are equivalent. While this equivalence holds without price uncertainty, we show that ad valorem taxes and specific taxes are “almost never” equivalent in the presence of uncertainty if we require equivalence to hold pathwise. Since we obtain this result under perfect competition, our analysis also provides a further rationale for why the equivalence must fail under imperfect competition.