This article explores the issues arising from the refusal of a respondent to pay its share of the advance on costs in an arbitration, the claimant’s substituted payment in respect of it, and the claimant’s entitlement to immediate reimbursement by the respondent before the final award is rendered and costs are allocated. The article will discuss the position under various institutional rules and draw a comparative analysis between them. It will also explore the legal basis of a claim for reimbursement, consider the approach that arbitral tribunals should adopt when granting relief, and examine a recent partial award in a Doha-seated International Chamber of Commerce (ICC) arbitration which ordered the immediate reimbursement of a substituted payment of the advance on costs.