Motivated by an ex situ TEM-investigation on the internal reduction of (Mg1-xCux)O [1], we have performed a XAS analysis on the precipitation formation in (Mg1-xCux)O after exposing the specimen to a(O2) = 6x10(-12) for several hours. As the reduction rate is slow, a detailed characterisation of the reaction path was elaborated for absorption measurements on quenched samples. In addition, a series of spectra was recorded applying the in situ XAS technique at the copper K-edge. The changes e.g. in the XANES region correspond well to those observed on the quenched specimens.