Remodelling protein nucleic acid interfaces is an important biological task, which is often carried out by nucleic acid stimulated ATPases of the Swi2/Snf2 superfamily. Here we study the mechano-chemical cycle of such an ATPase, namely the catalytic domain of the Sulfolobus solfataricus Rad54 homologue (SsoRad54cd), by means of fluorescence resonance energy transfer (FRET). The results of the FRET studies show that the enzyme can be found in (at least) two different possible conformations in solution. An open conformation, consistent with a recently reported crystal structure, is converted into a closed conformation after DNA binding. Upon subsequent binding of ATP no further change in conformation can be detected by the FRET measurements. Instead, a FRET detectable conformational change occurs after ATP hydrolysis and prior to ADP release, suggesting a powerstroke that is linked to phosphate release. Based on these data we will present a new model for the mechanochemical cycle of this enzyme. This scheme in turn provides a working model for understanding the function of other members of the Swi2/Snf2 family.