This paper examines the characteristics and relative strength of third-party sanctions in a series of experiments. The authors hypothesize that egalitarian distribution norms and cooperation norms apply in their experiments, and that third parties, whose economic payoff is unaffected by the norm violation, may be willing to enforce these norms despite the cost to themselves. The results show that almost two-thirds of the third parties punish violations of the distribution norm, and their punishment increases with the severity of the violation. Similarly, up to about 60% of third parties punish violations of the cooperation norm. This suggests that strong reciprocity, a concept in economics, extends to the sanctioning behavior of "unaffected" third parties. The experiments also indicate that second parties, whose economic payoff is reduced by the norm violation, punish violations more strongly than third parties. Questionnaire evidence supports the idea that fairness motives and negative emotions are determinants of third-party sanctions. The paper contributes to the understanding of social norms by studying the enforcement mechanisms and provides a method for studying the characteristics and content of social norms.This paper examines the characteristics and relative strength of third-party sanctions in a series of experiments. The authors hypothesize that egalitarian distribution norms and cooperation norms apply in their experiments, and that third parties, whose economic payoff is unaffected by the norm violation, may be willing to enforce these norms despite the cost to themselves. The results show that almost two-thirds of the third parties punish violations of the distribution norm, and their punishment increases with the severity of the violation. Similarly, up to about 60% of third parties punish violations of the cooperation norm. This suggests that strong reciprocity, a concept in economics, extends to the sanctioning behavior of "unaffected" third parties. The experiments also indicate that second parties, whose economic payoff is reduced by the norm violation, punish violations more strongly than third parties. Questionnaire evidence supports the idea that fairness motives and negative emotions are determinants of third-party sanctions. The paper contributes to the understanding of social norms by studying the enforcement mechanisms and provides a method for studying the characteristics and content of social norms.