Speech technologies, or technologies that recognize and respond to human speech, have recently emerged as a ubiquitous and cost-effective form of customer self-service (e-service). Although customer satisfaction is regarded as an important outcome of e-service interactions, little is known about users affective responses to conversational interactions with technology. Using a theoretical foundation derived from research in social cognition, interpersonal communication, psycholinguistics, human factors, and services marketing, two studies develop items for a speech interface usability scale, which is then used to examine interrelationships among individual differences (e.g., self-monitoring, need for interaction with a service provider, inherent novelty seeking), usability, comfort, and customer satisfaction. In the first study, speech and language experts' ratings of sixteen speech interfaces are used to identify six stimuli for the second study that represent high, average, and low quality. In the second study, participants listen to the six interfaces and provide ratings for the primary variables of interest. Results indicated that speech interface usability consists of four factors (User Goal Orientation, Customer Service Behavior, Verbosity, and Speech Characteristics). Usability items and individual differences predict affective responses to speech-based e-service. Implications of these findings for psychological and communication research and applied speech technology are described.