Active replication is a common approach to building highly available and reliable distributed software applications. The redundant nested invocation (RNI) problem arises when servers in a replicated group issue nested invocations to another server group in response to a client invocation. Automatic suppression of RNI is always a desirable solution, yet it is usually a difficult design issue. In this research, we propose a new determinism reference model based on accomplishing the verification process in a more systematic manner. The proposed determinism reference model consists of four levels, namely: ideal determinism, isomorphic determinism, similar determinism, and non-determinism. We consider a class of multi-threading CORBA environments to demonstrate the power of the proposed determinism reference model.