We examine communication via a corporate Twitter channel and its effects on corporate reputation. We identify the importance of user engagement and informedness in explaining corporate reputation and examine three design factors that likely affect user engagement in a corporate Twitter channel. We conduct an exploratory 2 x 2 x 2 experiment among Twitter users to collect data. We find that the depth of the relationship among users, the level of corporate involvement, and the purpose of the channel interactively influence user engagement. Our findings suggest that deeper relationships among users of a corporate Twitter channel lead to higher user engagement when the level of corporate involvement with the channel is high and when the channel has a specific purpose, but not when the level of corporate involvement is high and the channel has a generic purpose. Surprisingly, when the channel has a generic purpose, a high degree of corporate involvement actually decreases user engagement. This finding implies that, under certain circumstances, a lower degree of corporate involvement in a social media channel may be more desirable. We also find that channel credibility positively influences user informedness. This is the first study that examines the dynamics of communication through a corporate Twitter channel. It contributes to the previous research related to social media by identifying engagement and informedness as two major factors that influence firms' reputation. Our research can help marketing and social media managers to decide on channel design aspects, such as whether to require users to register with an identity or to allow anonymous participation, whether to allocate dedicated employees to respond to user requests, and whether to set up different channels for different purposes.