A nonpolymeric silsesquioxane is provided wherein at least one silicon atom of the silsesquioxane is directly or indirectly bound to an acid-cleavable substituent RCL. The silsesquioxane has a glass transition temperature Tg of greater than 50° C, and the RCL substituent can be cleaved from the silsesquioxane at a temperature below Tg, generally at least 5° C. below Tg. The remainder of the silicon atoms within the silsesquioxane structure may be bound to additional acid-cleavable groups, acid-inert polar groups RP, and/or acid-inert nonpolar groups RNP. The nonpolymeric silsesquioxane can be a polyhedral silsesquioxane optionally having one to three open vertices, such that the polyhedron appears to be a “partial cage” structure, or a macromer of two to four such polyhedral silsesquioxanes. Photoresist compositions containing the novel nonpolymeric silsesquioxanes are also provided, as is a method for using the compositions in preparing a patterned substrate.