An error has been detected in the methodology for estimating batch melt compositions from plagioclase and spinel lherzolites at mantle conditions presented by Kinzler and Grove (1992a,b) (hereafter referred to as K&G). In a mass balance calculation employed by K&G the melt fraction was not properly transformed from weight units to molar units, resulting in an overestimate of the effective melt fraction employed in the calculation of the Mg ♯ (Mg/2+>) of the melt, at melt fractions greater than 0.01. In the batch melts estimated by K&G, the abundance of FeO increases then decreases in the melt with increasing melt fraction from 0.01 to 0.20. Using the corrected methodology described here, the abundance of FeO in melts produced via batch melting increases as melt fraction increases from 0.01 to 0.20 (up to the point of exhaustion of clinopyroxene). The correction has essentially no effect on the compositions of melts produced via the near-fractional melting processes described in K&G because melt fractions employed in the near-fractional melting calculations never exceeded 0.01. In addition we apply the methodology of K&G to estimate the mantle temperature, average pressure of melting, and average extent of melting beneath a mid-ocean ridge segment from the FeO and Na2O content of a mid-ocean ridge basalt from that segment. ¿ American Geophysical Union 1993