The phase behavior of Ar-Kr gas mixtures adsorbed in porous Vycor glass and in bulk have been investigated using positron annihilation spectroscopy. For the bulk mixtures, we have determined the liquid/vapor coexistence region and the upper bound of the liquid/solid coexistence region. When the gas mixtures are adsorbed in Vycor, the phase transitions are shifted. It is found that the condensation transition shifts by a far greater amount than it does for pure gases. In addition, there is evidence of microbubble formation during the latter half of the pore-condensation transition for Kr and Ar-Kr mixtures but not Ar. These microbubbles persist for longer in the mixtures. The hysteresis at the gas/liquid phase boundary is much bigger for the mixtures compared to that of the pure gases. In particular, the desorption branch of the hysteresis loop is very broad and does not meet up with the adsorption branch over the temperature range of the present study. We found none of the usual evidence of pore blocking upon desorption for the mixtures. The position of the shifted phase boundaries for the mixtures in Vycor were determined and compared to those for the bulk.