<p>Rules in international arbitration can have various characterisations. Some are ‘substantive’ and others ‘procedural’, determined by direct party agreement or subject to tribunal discretion. Given that the proper meaning of these concepts tends to be conflated and interpreted without rigour, this paper argues that, subject to contrary mandatory provisions in the <i>leges arbitri</i> and in the absence of direct party agreement on procedure, tribunals should follow the procedure-substance characterisation found under the applicable substantive law selected by the parties. If substantive law deems a rule to be procedural, tribunals are under no duty to apply the rule. Instead, tribunals can rely on their discretion to determine the appropriate course of action. Conversely, if substantive law characterises a rule as substantive, tribunals must apply the rule, despite the scope for challenges to arbitral awards on grounds of errors of law is minimal. In doing so, the article distinguishes how the issue is approached in arbitration compared to general private international law and acknowledged that the true meaning of ‘procedure’ and ‘substance’ requires a principled case-by-case analysis.</p>