Experimental phase equilibria have been investigated on three medium-K silicic andesite (60--61 wt% SiO2) samples from Mount Pel¿e at 2--4 kbar, 850--1040 ¿C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni-NiO buffer) to NNO+1 and NNO+2 to NNO+3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875--950 ¿C and 5--7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876 ¿C. At 900 ¿C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pl+mt is replaced by am+mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O-rich part of the diagram at 930 ¿C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changing fO2. Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl+opx+mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk-alkaline magmatism (approximately NNO+1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents. ¿ 1999 American Geophysical Union