The piezo XAFS technique in combination with an in situ cell has been used for the investigation of fast solid-solid transformations with millisecond time resolution. The technique records X-ray absorption spectra (XAFS) in a continuous mode by making use of piezo tilt tables in the X-ray monochromator, which are actuated by an oscillatory high voltage. The application of this technique in the field of solid-state chemistry and catalysis is illustrated by investigations of the X-ray absorption near-edge structure (XANES) of the Cu K edge during the autocatalytic reduction of a Cu/ZnO methanol catalyst and the spontaneous decomposition of (NH4)(2)Cr2O7 to Cr2O3. The reduction of Cu(II) to Cu(0) occurs within a few seconds. The existence of an intermediate Cu(I) phase could be revealed by the analysis of the preedge position and the white line intensity, but the Cu(I) intermediate is very unstable. The spontaneous decomposition of (NH4)(2)Cr2O7 occurs within about 30 s acid shows after a short acceleration period a rather constant reaction rate and a quite long decay period.