Whereas existing learning environments on the Web lack high level interactivity, we have developed a human tutor-like tutorial conversation system for the Web that enhances educational courseware through mixed-initiative dialog with natural language processing. The conversational tutoring agent is composed of an animated tutor, a Latent Semantic Analysis (LSA) module, a database with curriculum scripts, and a dialog manager. As in the case of human tutors, the meaning of learner's contributions in natural language are compared with the content of expected answers to questions or problems specified in curriculum scripts. LSA is used to evaluate the conceptual matches between learner input and tutor expectations, whereas the dialog manager determines how the tutor adaptively responds to the learner by selecting content from the curriculum script. The integration of available courseware with the tutorial dialog system guarantees the reusability of existing Web tutorials with minimal effort in the modification of the curriculum script and LSA module. This development thereby simplifies the change into more valuable Web based training courseware.