SummarySecond derivative diffuse reflectance spectroscopy (DRS) in the visible range has been used to characterize changes in colour and identify the nature of Fe oxides which withstand reduction during experimental yellowing of reddish materials. It is accepted that haematite dissolves preferentially and faster than goethite, and Al‐substitution controls the dissolution kinetics of Fe oxides. However, DRS has shown that haematite is more resistant than predicted and that some Fe‐oxides, probably trapped within kaolinite particles, are inaccessible to solvents. DRS allows the nature of dissolved phases at each deferration step to be determined and changes in Al‐content of residual phases throughout deferration to be followed. It also demonstrated that Helmholtz coordinates correlate very well with changes in Fe‐oxide mineralogy and are preferable to redness ratings when monitoring differential dissolution of Fe oxides through colour measurements. DRS is a powerful and sensitive technique for monitoring the dissolution of Fe oxides i