EU Competition Law has various evidentiary rules and presumptions relating to the existence of collusive activity. We consider the effect of these rules and presumptions on economic efficiency, and highlight two forms of inefficiency associated with existing adjectival law. Firstly, both under and over regulation of the inferential process introduce error costs flowing from erroneous inferences. Due to the inherent nature of collusive activity, cases involving Competition Law are likely to invite a particular type of erroneous inference - the inference of collusive activity from the parallel conduct of firms. Secondly, we illustrate a different type of inefficiency that arises in Competition Law. Since evidential rules and presumptions have the potential to influence the behaviour of firms in a collusive setting, the over-regulation of the inferential process has the potential to ameliorate the sustainability of a collusive equilibrium, undermining a key rationale of substantive Competition Law. We set out three specific areas of adjectival law which exhibit these forms of inefficiency, and argue for appropriate reformation of the law.