Volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions. Applying geodetic techniques to monitoring activity involves interpretation using deformation models. Usually, the observed changes of the deformation and gravity fields are interpreted separately, not in a joint inversion. It can be difficult, if not impossible, to interpret the data coherently or correctly in terms of the characteristics of the intrusion or the deflation derived from the gravity changes with purely elastic models, as in the case of Mayon Volcano, Phillipines. We show that elastic-gravitational models can be used to interpret these cases simultaneously leading to a result that is more plausible on the basis of the available information. Thus, we may need to change the philosophy normally used to interpret geodetic observations. Interpretation as proposed in this work can significantly improve the possibility of predicting future eruptions. ¿ 2001 American Geophysical Union