The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X-ray absorption near-edge structure spectroscopy at the Fe-K-edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe⁰ with respect to oxidized iron (Fe_ox = Fe²⁺ + Fe³⁺) or the iron distribution in some specific phases (e.g., Urey-Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe⁰, Fe²⁺, and Fe³⁺. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe²⁺) + 3(Fe³⁺)]/[Fe_TOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe³⁺ to Fe²⁺ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe³⁺ is present, together with Fe²⁺ and sometimes Fe⁰. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least-altered CR chondrites, the fraction of Fe³⁺ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine-grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).