Recent improvements in speech technology have made synthetic speech a viable I/O alternative. However, little research has focused on optimizing the various speech parameters which influence system performance. This study examined the effects of speech rate, message repetition, and the placement of information in a message. Briefly, subjects heard messages generated by a speech synthesizer and were asked to transcribe what they had heard. After entering each transcription, subjects rated the perceived difficulty of the preceding message, and how confident they were of their response. The accuracy of their response, system response time, and response latency were recorded. Transcription accuracy was best for messages spoken at 150 or 180 wpm and for messages repeated either twice or three times. Words at the end of messages were transcribed more accurately than words at the beginning of messages. Response latencies were fastest at 180 wpm with 3 repetitions and rose as the number of repetitions decreased. System response times were shortest when a message was repeated only once. The subjective certainty and difficulty ratings indicated that subjects were aware of errors when incorrectly transcribing a message. These results suggest that a) message rates should lie below 210 wpm, b) a repeat feature should be included in speech interface designs, and c) important information should be contained at the end of messages. / Master of Science