In the Brđanin and Krajišnik judgments, the ICTY Appeals Chamber found that the accused may incur criminal liability for crimes carried out by low-level non-joint criminal enterprise (JCE) physical perpetrators who are used as tools or are otherwise instrumentalized by JCE members other than the accused to carry out the crimes of the common plan. Similarly, the Appeals Chamber held that the accused may also incur liability for crimes of excess committed by non-JCE perpetrators. However, as for the precise nature of liability in such cases, no clarification was provided. From this ambiguity the author detects an inclination on the part of the Appeals Chamber to construe JCE not only as an expression of commission but also of complicity. The author then identifies and elaborates the theoretical difficulties that this construction would instil in the traditional doctrine of JCE as formulated by the Tadić Appeals Chamber and the international law of individual criminal responsibility. He suggests that awareness of these very problems of theory is the reason behind the conspicuous ambiguity in Brđanin and Krajišnik as to the nature of JCE liability.