Cooperation among people who are not related to each other is sustained by the availability of punishment devices which help enforce social norms (Fehr and Gächter, 2002). However, the rationale for costly punishment remains unclear. This paper reports the results of an experiment investigating inequality aversion and negative emotions as possible determinants of punishment. We compare two treatments of a public good game, one in which costly punishment reduces the immediate payoff inequality between the punisher and the target, and one in which it does not affect inequality. We show that while inequality-aversion prevents some subjects from punishing in the equal cost treatment, negative emotions are the primary motive for punishment. Results also indicate that the intensity of punishment increases with the level of inequality, and reduces earnings inequality over time.