We present Sr and Nd isotopic data for 18 basanites, and chemical and isotopic data for five associated kaersuties from the Ventura and Santo Domingo volcanic fields in the Mexican state of San Luis Potos¿. All isotopic data fall in or near the ''mantle array.'' The kaersuities have the mos depleted isotopic compositions (low 87Sr/86Sr and high ϵ143) but are less depleted than mid-ocean ridge basalts (NMORB). ?Basanite isotopic compositions are distributed bimodally, with most samples in the more depleted mode, which is near the high ϵ143 and low 87Sr/86Sr end of the compositional range observed for ocean island basalts (OIBs). In a companion paper, Luhr et al. <1989> find chemical variations in these volcanic rocks that suggest partial melting could be responsible for the principal chemical trends, but the variability of isotopic compositions precludes any simple one-component model. 87Sr/86Sr, but not ϵ143, is found to correlate with these chemical trends. SiO2 is lowest and normativene + lc, incompatible element concentrations and light versus heavy rare Earth elment ratios are highest in samples with the lowest 87Sr/86Sr values. Crustal contamination may be significant for a few of these samples, but we conclude that the principal isotopic trends arise in mixing of at least three mantle resrvoirs: a depleted component analogous so the MORB reservoir, a St. Helena type component, and a third component which affects Sr composition substantially more than Nd composition. The third component carries relatively radiogenic Sr and appears to correlate with degree of partial melting, suggestive of a hydrous component, possibly derived from a subducted slab, such as has previously been invoked in studies of island arcs. ¿ American Geophysical Union 1989