Introduction. Type B CAIs are often enriched in the heavy isotopes of magnesium and silicon, suggesting that a fraction of these elements was lost by evaporation [1]. Detailed comparisons between compositions calculated for a cooling solar composition gas and recent estimates of the bulk composition of Type B CAIs further suggests that evaporation played a significant role in that the Type B CAIs contain significantly less of the relatively volatile magnesium than calculated [2]. The silicon content, on the other hand, is not as useful a measure of evaporation because solids condensing from a solar composition gas in the temperature interval over which the typical mineral assemblages of the Type B CAIs are stable can have a large range in silicon content not much different from that of the CAIs themselves. For this reason we will focus on the behavior of magnesium during the evaporation of a Type B CAI-like melt. The proposition that the elemental and isotopic compositions of the Type B CAIs have been measurably affected by evaporation can be quantitatively tested, given the right data and a realistic representation of the relationship between elemental and isotopic fractionation during evaporation. For example, given data on the heavy isotopic enrichment of magnesium and/or silicon of a set of Type B CAIs one could calculate the amount of magnesium and/or silicon that must have evaporated to account for the isotopic data and then check whether the observed composition of the CAIs, adjusted to take this loss into account, corresponds to plausible condensates from a solar composition gas (see [3] for such an attempt). A key issue is the appropriate relationship one should use for relating elemental and isotopic fractionations in the specific case of magnesium evaporation from a molten Type B CAI-like composition. The kinetics of evaporation from a condensed phase into a gas of sufficiently low pressure is given by the HertzKnudsen equation,