An explicit record of design rationale is an important requirement for effective design support. Such a record may be defined as the information supporting or explaining the decisions made by a designer when generating an artifact. Design rationale can be used to (a) improve the documentation of the design process, (b) verify the design methodology and the resulting design artifact, (c) provide support for the analysis and explanation of the design process, and (d) enhance the re-usability of parts of the design artifact. This paper describes an extension to KBDS, a prototype design support system for conceptual design of chemical processes. The extension makes use of an IBIS representation to record the design rationale and integrates it with the design history maintained by KBDS. Thus, KBDS-IBIS records in an integrated and prescriptive form the specification of the design artifact, the history of its evolution during the design process, and the designer's rationale.