Course management systems such as Blackboard include two distinct modes of web-based discussion, -- real-time and delayed-timed. Both communication formats are promoted as having both advantages and disadvantages with respect to student learning and satisfaction. Ninety-three university students discussed two case studies using two real-time Blackboard chat and two case studies using delayed-time Blackboard discussion. Mastery of case study content was determined via objective examination items and student preference for CMC mode was surveyed. An ABAB research design allowed for comparison of student achievement across communicative conditions (e.g., A = delayed-time discussion followed by an examination; B = real-time discussion followed by an examination). In every contrast of real-time and delayed-time CMC mode, student achievement on objective examination items was equivalent. Many students (43% of the sample) reported the perception that real-time CMC facilitated their learning more than delayed-time CMC. However, such students were at a learning disadvantage, as measured by objective examination test performance, when using that CMC mode. Consequently, results of the current investigation do not support the educational implementation of real-time CMC without corresponding implementation of delayed-time CMC and mechanisms to ensure that students do not rely entirely on real-time text-based communication to master course content.