The research described in this paper was part of an investigation into human and task characteristics of jobs in more-or-less automated systems. An attempt was made to develop appropriate criteria of control performance for a simulated process control task. A field study was used to reveal possible problems in stating control goals and to assess some aspects of control behaviour. Eight operators controlling a real distillation plant were interviewed about their control behaviour. A decision scheme was developed and used to structure questions concerning control goals. The results showed that operators controlling the same process differ widely in control goals due lo a lack of information concerning technical and economical aspects of the process. These findings stress the need for a careful defined control goal when experimenting with a laboratory process.