Group IAB irons probably come from a parent body that partially differentiated into a sulfur-rich molten core and a primitive silicate outer layer in which only minor or localized melting occurred. Although the complexity of meteorites from this parent body or bodies (irons with and without massive silicate inclusions, stony meteorites like Winona, sulfide-rich meteorites like Mundrabilla, etc.) implies a complicated history, the partial differentiation model is consistent with most properties of these meteorites. The behavior of five model parent bodies of different composition during heating to 1400 K (the temperature recorded in many IAB silicates) shows that meteorites like IAB irons can form by this process. Sulfide-rich parent bodies differentiate into a liquid, S-rich core and a primitive, metal-bearing mantle around 1230 K, but in parent bodies lower in sulfide abundance this process may be delayed until minor melting of silicates occurs (1370¿30 K). In most cases any silicate melt that may have formed resolidified in situ. The sulfide melt fractionally crystallized along the (Fe,NI)-FeS cotectic. Some melt was injected into silicates to form Copiapo-type breccias. Considerations of graphite-metal-silicate equilibrium and the retention of noble gases suggest that brecciated IABs formed at depth inside a moderately large (a few tens to a hundred kilometers) parent body. The correlation of I-Xe ages with Ni content is interpreted as the result of an excess 129Xe component associated with the S-rich melt, which increases with melt evolution.