Three experiments are reported which examined the relative effects of geometry and object-specific function on the comprehension of the spatial prepositions in and on. The first experiment manipulated the height of a located object on top of a pile of other objects in containers which were primarily containers of solids (e.g., a suitcase) or liquids (e.g., an aquarium). The association between located object and reference object was also varied (by using different types of objects as located objects). In was found to be more appropriate to describe the same object in containers of solids compared to containers of liquids, although no effects of located object association were found. Experiments 2 and 3 manipulated similar variables with supporting surfaces rather than containers, and examined the effects of functional control on the comprehension of on. The studies provide evidence for the importance of functional relations on the comprehension of on. In addition effects of located object association were found, but only when there was no clear evidence for the absence or presence of functional control. The results are discussed in relation to the differential effects of object knowledge on the comprehension of spatial prepositions.