This study used resistivity probes for the dynamic detection of liquid-vapor phase change inside wet paper webs undergoing impulse pressing. The probes were made from thin copper wires insulated with a high-temperature polymeric material (overall diameter-of 50 mu m). The gap between the exposed ends served as the probe tip. The validity of this technique in impulse pressing experiments was demonstrated in two ways: (1) tests with both dry and wet sheets showed that the probe response was unaffected by the fiber network itself; and (2) heated trials with low applied pressure and long residence times (> 1 s) clearly showed the propagation of a vapor front through the sheet, as predicted by theory. Impulse pressing, experiments were subsequently conducted with a platen press under realistic operating conditions (i.e. surface tempera tures up to 300 degrees C, nip residence times of approximately 25 ms, and maximum applied pressure around 4 MPa). Results show that steam was detected inside the sheet only upon unloading of the hot nip for the layers closest to the heated surface.