Ultrasonic P wave velocity, effective porosity, grain and bulk density, residual water saturation and petrography of 46 tuff samples from the epithermal system of Creede, Colorado, were analyzed with stepwise multiple curvilinear regressions to predict dry bulk density, P wave velocity (Vp), and acoustic impedance (I). Changes in porosity best predict variations of all three parameters; residual water saturation is significantly correlated with Vp and I for coarse-grained matrix tuffs; and high aspect ratio cracks have a small but significant ability to predict Vp and I. The importance of porosity suggest that seismic and gravity work could be used to outline porosity-related structures of crucial importance to our understanding of Creede's epithermal system. In areas where lithologies are comparable to our samples, seismic velocity or acoustic impedance measured remotely with seismic surveys may yield estimates of porosity and provide important quantitative information on rocks that cannot be studied directly through a comprehensive drilling program. ¿ American Geophysical Union 1989