14 January 2014The ambient tax is often considered as an effcient instrument to achieve a first best outcome of ambient pollution when the regulator is less informed than the polluters. Since the ambient tax was never implemented in the field, empirical evidence is missing. Available experimental findings provide mixed evidence: effciency is higher under external damage, i.e. if ambient pollution affects non-polluters (Spraggon, 2002, 2003) than under internal damage, i.e. if ambient pollution affects polluters themselves (Cochard et al., 2005). Since these two types of experiments relied on very different designs, it is worthwhile to compare them under a common experimental design. Our main finding is that the ambient tax is equally effcient under external damage than under internal damage.