Instant messenger is being rapidly deployed in the workplace. Current studies largely focus on the adoption of IM and how IM is used. Little research has been conducted to understand the potential impact of using IM in the workplace. This paper contributes to the literature on Instant Messaging and task performance by theorizing and empirically testing how some technological features of IM could intertwine with social characteristics and jointly influence user task performance and perceived workload. We also study the impact of perceived task complexity on task performance and perceived workload. The effect of interruption on task completion time is dependent upon the hierarchy level of message sender. Interruptions from a supervisor were found to reduce primary task completion time whereas interruptions from a peer increase primary task completion time. In addition, interruptions from a supervisor aggravate the negative impact of interruptions on task quality.