This paper demonstrates the use of the Extended Equal Area Criterion (EEAC) to determine if islanded power systems remain transiently stable after the network has been split. Controlled system islanding has been proposed as a method of last resort to avoid a system-wide blackout. Such methods are usually based on some minimum cutset and/or minimum power disturbance criterion but do not specifically take transient stability into account. By developing an a priori transient stability indicator, any splitting methodology can be made more robust as the proposed islands will satisfy static (load, generation balance) as well as transient constraints. If the proposed cutsets result in islands that are transiently unstable, this can be fed back to the splitting methodology and a new cutset and islanding configuration proposed.