In situ S K-edge XANES experiments were carried out on second-generation SOx trapping materials under oxidizing and reducing conditions. The experiments clearly show that the strong release of SO2 under rich conditions at plug flow conditions is caused by the facilitated reduction of sulfite species on Pt. In the absence of Pt the sulfite species were stable under reducing conditions, while maintaining a similar total SO2 uptake capacity. Thus, SOx trapping materials without a noble metal are a clearly better option. The enhancing effect on the SOx storage process of water present in the gas mixture is attributed to the formation of a higher sulfate fraction in the samples. The application of the in situ S K-edge XANES technique clearly reveals new information and insights on the behavior of the sulfur in the trapping process compared to that from the ex situ measurements and is therefore essential for designing new SOx trapping materials.