The isothermal cell edge and volume compressions of α quartz were measured to 12-GPa (120 kbar) hydrostatic pressure by using the modified Jamieson technique. Detailed listings are given of the relative volumes of Al, NaF, and NaCl, the pressure indicators, correlated with pressures to 13 GPa. The measured relative cell edges and volumes of α quartz to 12 GPa are combined with the 0.3-GPa elastic moduli data of McSkimin et al.(1965) to give the following relative cell edge and volume fits: a/a0=1-9.815¿103P+8.155¿10 2P2-7.779 ¿10-5P+3.288¿10-6P4; c/c0=1-7.306¿10-3P +7.875¿10-4P2-6.685¿10-5P3+2,362¿10-6P@qL 4; ust=3.7437+1.825upt-0.152upt 2,@qL upt0.5 km/s. Here V/V0= (ust -upt)/ust, P=&rgr;0ustupt, and &rgr;0=2.647 g/cm3. The equilibrium shock compression Hugoniots calculated from the isothermal compression fits are us=3.758 +1.828up-0.131up 2, up0.45 km/s, where us is the shock velocity and up is the particle or mass velocity behind the shock wave.