Alaboratory experiment was used to investigate the effects on decision maker performance of using geographic information system (GIS) technology as a spatial decision support system (SDSS). The research examined two independent variables: task complexity (i.e., low, medium, and high complexity, and SDSS use (i.e., no SDSS versus SDSS support). Professionals who are experienced decision makers completed a site location task that required decisions to be made based upon spatially-referenced information. The results confirm the hypotheses and show that SDSS use and task complexity both have an important impact on decision quality and solution time. The study builds upon and extends image theory as a basis for explaining efficiency differences resulting from differing graphical displays of spatial information