This paper evaluates the strengths and weaknesses of the four mainstream choice of law approaches in determining the governing law of the arbitration agreement (‘lex contractus arbitratus’) in the absence of any explicit appointment by the parties. These include: (1) lex contractus; (2) lex loci arbitri; (3) the a-national approach; and (4) the validity approach. This paper determines that not only does the lex loci arbitri approach produce great commercial benefits by ensuring the enforceability of the arbitral award pursuant to the New York Convention (NYC) and by mitigating legal complexity and unpredictability, but is also the most doctrinally compelling approach considering the doctrine of separability and close connection. This paper concludes with the proposal that an ideal approach ought to begin with the presumption of lex loci arbitri, with possible countervailing factors such as the parties’ true intention as evinced through parol evidence and the choice of law’s effect on the validity of the arbitration agreement.