This paper describes an experimental study aimed at investigating the learning effectiveness of a Web-based course called "Academic English" (EAP) for Japanese learners of English. The main focus of the study was to examine the form, function, and impact of interaction in the course. Twenty university-level EFL students participated in this experiment. They were randomly assigned to two treatment groups: a controlled group with "inter-personal task" treatment and an experiment group with "intra-personal task" treatment. Regarding effectiveness of the interactivity dimension in language acquisition, there was one independent variable, "type of treatments," and two dependent variables, "achievement" and "attitude measurements". The achievement variable was used to address the extent of learning due to the two treatment conditions. The attitude variable was used to address the degree of motivation towards a Web-based communication platform as well as the enforced interaction functions. Statistical analyses revealed no significant effect of treatment on either comprehension or general L2 development, but significant differences were found with respect to language interaction for task completion under different task treatments. Therefore, it was concluded that this Web-based course was well designed to maximize the students' language learning experience as well as to improve their language abilities in English. Further research is needed to examine the notion of self-repair in students' production in the target language. (Contains 6 figures, 12 tables and 2 notes.)