The synergism of steam reforming and photocatalysis on converting the tar model compound toluene was investigated in a microwave (MW)-metal discharge reactor, using anatase TiO2 as photocatalyst and N-2 and N-2+Ar as carrier gas. Unlike dry-state cracking that generates noticeable soot, MW-metal discharge steam reforming can effectively eliminate soot formation and promote the conversion of toluene into permanent gases. The toluene conversion in steam reforming can be further enhanced by employing photocatalyst. However, the photocatalytic performance largely depended on the carrier gas and humidity. Compared with N-2, the introduction of Ar into N-2 intensified the UV light emission to trigger photocatalytic degradation of toluene. The toluene conversion efficiency under the synergetic effects of photocatalysis and steam reforming reached 98% when Ar/N-2 = 1/5 (v/v) was used as the carrier gas with a moderate humidity of 38%. However, toluene was less effectively photodegraded as humidity increased high (>60%), mainly attributed to competitive adsorption between toluene and water molecules on the active sites of photocatalyst as well as the reduction in discharge intensity with humidity increase. The disclosure of the plasma and photocatalytic effects of MW-metal discharges is expected to aid further research in the field of microwave chemistry. (C) 2018 Elsevier Ltd. All rights reserved.