One of the central questions of spatial reasoning research is whether the underlying processes are inherently visual or spatial. The article reports a dual-task experiment that was conducted to explore the visual and/or spatial nature of human spatial reasoning. The main tasks were inferences based on a spatial version of the interval calculus introduced by Allen (1983). The secondary tasks were presented visually or acoustically, and were either spatial or non-spatial. The results indicate that spatial reasoning is mainly based on the construction and inspection of spatial layouts, whereas no evidence of the involvement of visual representations and processes was found.