Video display units have penetrated into modern control rooms, replacing conventional instrumentation as a means of information presentation to the operators. In four experiments simulating a small chemical plant process, several process variable and presentation characteristics were investigated. The experiments focused on the early detection of ‘oncoming off-normals’ (ocons) — deviations in the process that have just begun to appear. The results show that the way information is presented and some characteristics of the signals presented affect detection performance. It is also suggested that the presence of historical information, in the form of trends, seriously hampers the early detection of oncoming off-normals.