The Carleton University Honeywell Level 66 computer system has been modelled by a closed queueing network consisting of a central CPU with disks and front end processors as the input-output devices. Terminal input proceeds through a front end processor. Data gathered from the system was used to find the mean device service times and the inter-device branching ratios, the parameters required to evaluate the system models. The models represent the system behaviour quite well. A comparison between the Honeywell system and the University's older Xerox system indicates that the two systems are roughly equivalent with the Xerox system having an advantage in the number of online users which may be supported for a reasonable response time. The models suggest that the CPU is the bottleneck and that a dual CPU or faster CPU would cause a considerable improvement.