Article 25.2 of the Spanish Arbitration Act gives arbitrators the power to decide on the taking of evidence «even ex officio». By contrast, Article 216 of the Spanish Law on Civil Procedure provides that the courts may only rule «on the basis of the evidence produced by the parties». Some scholars have pointed out that this unequal approach to the evidentiary initiative by arbitrators and judges would be an unjustified contradiction. This paper aims to show that this contradiction is merely apparent. We will first explore the reason why ex officio evidence is widely accepted in arbitration, and we will dismiss some widespread but, in our opinion, erroneous propositions. Based on that analysis, it will be scrutinised the nature of this power and the limits that must be observed when exercising it.