Modern communication technologies have modified the tradition of instructor's office hours in numerous ways. This article explores the use of Instant Messenger (IM) software in the context of "online office hours". The authors discuss strengths and weaknesses of IM interactions for instructor/student communication, and examine a sample of such communications from two graduate classes to determine the types of content found there. The authors propose a categorical structure drawn from this content which Contains three major dimensions, two of which have sub-dimensions; the purpose of the structure is to better understand the potential pedagogical use of TM. The "24 hour professor syndrome" is a real hazard for instructors making increasing use of any type of online interactions with students. This article helps to understand how better to use computer mediated communications to improve instructor/student interactions through integrating IM into online office hours, while remaining aware of the potential disadvantages of failing to establish a boundary defining periods of instructor availability. (Contains 1 chart.)