As many readers will be aware, ad hoc arbitration is not permitted by mainland Chinese law. This rule only applies, however, to arbitrations with a mainland Chinese seat. Chinese courts are required to enforce foreign ad hoc commercial arbitration awards subject to the usual New York Convention exceptions (assuming that the dispute is "foreign related" and, as such, arbitrable outside mainland China in the eyes of Chinese law).
There has, however, in the past been some confusion in some quarters as to whether Hong Kong ‘ad hoc’ arbitration awards will be enforced by the mainland Chinese courts. This has now been dispelled by a reply issued by China's Supreme People's Court to a query from the Secretary of Justice of Hong Kong, which confirmed that Hong Kong ad hoc awards are enforceable in the Mainland (subject to the same exceptions which apply to institutional awards). A copy of the reply is available here.