A laboratory suction box was built to study the dynamic thickness deformation of a fibre web during a suction pulse. Both the pressure in the suction box and the duration of the suction pulse are adjustable in ranges of 0 to -40 kPa and 50 ms to 10 s, respectively. During and after the suction pulse, the web thickness and the suction pressures can be measured. Furthermore, the air flow through the web can be calculated after the experiment. The first evaluation of the performance of the laboratory suction box shows that the web is considerably compressedaaaa especially during the initial phase of the suction pulse. This indicates that web compression is an important dewatering mechanism. After the suction pulse, a rapid and significant web expansion was observed, implying that considerable rewetting probably occurs.