Is it true that procedural and administrative rules and matters can never become part of customary international law?

Question

Is it true that procedural and administrative rules and matters can never become part of customary international law?

Answer 1

Is it true that procedural and administrative rules and matters can never become part of customary international law?

Not exactly. At some level it might, but only at a very high level of generality.

For example, customary international law might require some sort of "due process" or "notice and hearing" in some kinds of situations, before taking some kind of consequential action.

But it wouldn't spell out just what was involved in that, with details like whether it had to be before a single judge, panel of judges, or jury; whether the person presiding had to have some sort of legal training; what kind of notice has to be given; what deadlines and timeline need to be involved; what rules would apply to presenting evidence; and where a hearing needed to be conducted (or whether it had to be in person).

Similarly, customary international law might recognize some sort of evidentiary privilege for lawyers or diplomats, might recognize that defendants should be presumed innocent, or might recognize that at some level someone is incapable of having criminal liability (e.g. not a two years old, or when in advanced dementia) at least in extremely clear cases. But again, it would lack details.

Answer 2

The accepted rule of recognition for customary international law (general but not necessarily universal practice, and opinio juris, namely the belief that such practice amounts to a legal obligation) does not preclude in principle a procedural rule from attaining the status of customary international law.

However, given the varied and context-sensitive application of any procedural rule, it has been acknowledged that it may be difficult for a procedural rule or practice to be a sufficiently "general practice" among states to become customary international law.

Second, where a procedural rule is of such importance that it becomes generally practiced, the line between procedure and substance blurs. E.g. Would giving someone a hearing before deportation be a mere procedural rule? Or would it be a substantive right? Would a requirement for X days notice before a variation to trade practices be a procedural rule or a substantive right of the affected state? My point here is that were a procedural rule recognized as customary international law, that recognition may cause the rule to be viewed as substantive rather than merely procedural.