The results from measurements of extensive parameters (steam output and exhaling surface area) taken at the crater fumarolic field of Vulcano Island (Aeolian arc, southern Italy) are reported along with a detailed description of the measuring method. The steam emission rate increased by 1 order of magnitude during the observation time (1983--1995), while its concentration in the released gases was only slightly changed. During the same period the total exhaling surface expanded from 50 m2 to more than 2400 m2 and evolved following preferential trends that coincide with the main tectonic structures of the island. The observed peaks in steam output show a positive correlation with episodes of volcanic activity unrest. The temporal increase of steam output and the observed development in the exhaling surface areas are consistent with a volcano-tectonic triggering of the exhaling activity. Particularly, the peaks in the steam output are interpreted as a consequence of degassing from an active magma, slowly moving toward the surface. ¿ 1998 American Geophysical Union