The management of complexity in large systems is traditionally focused on the modeling and management of classes and hierarchies of classes. In order to improve the compositional flexibility in large systems, this paper turns the focus on objects rather than classes. We will demonstrate that a more powerful object system can ease the development of large systems and can improve the degree of code reuse. The paper introduces a new object-level language construct, per-object mixins, for object-based system composition. It is implemented in the scripting language XOTCL, which is an extension of MIT’s OTCL. Per-object mixins extend the method chaining mechanism of OTCL with the ability to mix classes into the precedence order of an arbitrary object. Per-object mixins can be used to implement state-specific behavior changes in a clean way. We present per-object mixins as a general approach to hide object specifics from client objects transparently.