Homogeneous chromium catalysts for the selective conversion of ethene to hex-1-ene are formed from Cr(III) reagents, aminothioether ligands of the type HN-(CH 2 CH 2 SR) 2 , and aluminum reagents. In this study, the early activation steps are investigated by EPR, UV−visible, and Cr K-edge XAFS spectroscopy; rapid stopped-flow mixing and a freeze-quench allows good quality EXAFS analysis of a species formed in ∼1 s of reaction. This is shown to involve reduction to Cr(II) and deprotonation of a NH group of the auxiliary ligand. This 4-coordinate metal center may act as precursor for the coordination of ethene and subsequent selective oligomerization. T he selective trimerization of ethene to hex-1-ene catalyzed by chromium provides improved atom efficiency over chain-growth processes. 1,2 Highly effective catalysts for this trimerization have been reported from precursors of the type [CrCl 3 {R-SN(H)S-R}] 1 (R-SN(H)S-R = HN(CH 2 CH 2 SR) 2 with methylaluminoxane (MAO) as promoter. 3 Such activation of precursors has generally been investigated via the crystallization of reaction products. Reaction of 1 (R = Cy) with 10 AlMe 3 effected partial methylation to a Cr(III) product [(μ-Cl) 2 {CrMe(R-SN(H)S-R)} 2 ][AlMe 3 Cl] 2. 4 However, with iso-butylaluminoxane reduction to a Cr(II) relative, [(μ-Cl) 2 {Cr(R-SN(H)S)-R} 2 ][AlCl 2 i Bu 2 ] 2 was isolated instead, and the preferred oxidation state depends upon the alkylaluminum. 5 We have previously utilized Cr K-edge X-ray absorption spectroscopy to explore activation steps in solution. 6