A reversible ordered state induced by the combined action of above band-gap light and temperature in a-Si1-xGex layers with 0≤x≪0.38 is reported. The experimental conditions for the observation of this effect are described, as well as the main phenomenological aspects. The illumination of these layers with above band-gap light from an Ar+ laser in the temperature range from 110 to 190 °C induces changes in the Raman spectra that reveal the formation of small ordered clusters (only a few lattice parameters large). The ordered state is not metastable in the sense reported for other structural transformations in these materials, once it was formed it disappears by both cooling down and heating up beyond the above-mentioned lower and upper temperature thresholds. The effect reappears when the sample is settled at a temperature within the above range under illumination. The analysis of the Raman spectra allows to estimate the crystallite size, the amount of internal stress and the fraction of ordered material. The possible mechanisms governing this effect are discussed. © 2004 American Institute of Physics.