Critiquing systems are a type of active, knowledge-based design support system. They propose to positively influence designers’ cognitive processes by pointing out potential problems and contentious issues while designers work. To investigate the effects such systems have on the activities of professional designers, a design environment containing a critiquing system was designed, built, and evaluated for a specific area: phone-based interface design. Four professional designers were observed using the environment to solve realistic design tasks. Our protocol analyses indicate that such systems do influence the behaviour of designers, but often indirectly. Designers were observed anticipating the activity of the system and taking preventative steps to avoid it. Differential effects depending on the designers’ level of domain experience were also observed. Overall, the system was better suited to the needs of highly experienced designers.