Art. 829, co. 3, cod. proc. civ., as amended, applies to all arbitral proceedings commenced after the entry into force of the amending statute; however, in order to establish whether a challenge for breach of the rules of law governing the merits of the dispute is admissible, the law to which that provision refers must be identified as the law in force at the time the arbitration agreement was concluded. Where the arbitration agreement was concluded before the reform, absent any stipulation by the parties, a challenge to the award for non-observance of the rules of law is admissible, unless the parties had authorised the arbitrators to decide ex aequo et bono or had declared the award not open to challenge. The conferral on the arbitrators of the power to decide ex aequo et bono is not incompatible with arbitration, nor is the correct exercise of that power open to review in setting-aside proceedings. An arbitration clause in the articles of association providing for a decision ex aequo et bono cannot be unilaterally modified by an individual shareholder by the mere expression of a contrary intention in the arbitration, since the specific procedures prescribed for amending the articles must be observed. It does not amount to a breach of the adversarial principle (contraddittorio) for the arbitrators to choose to obtain in writing the clarifications of the tribunal-appointed expert, without fixing a hearing for the attendance of the party-appointed experts, where the parties were in any event afforded the opportunity to submit observations on the expert’s report.