We are examining Li, Be and B abundance variations in mafic lavas along strike in the Trans-Mexican Volcanic Belt (TMVB), building on previous undergraduate research efforts focused on the Colima Graben and two sites in the Michoacan-Guanajuato Volcanic Field (MGVF; Fuller et al 2003; Ryan et al 2005). Our past work indicated that both differentiated calc-alkaline and alkaline (i.e., lamprophyric) TMVB lavas show evidence for assimilation of crustal materials variably enriched in boron (i.e., Hochstaedter et al 1996), while more mafic lavas appear to preserve signatures reflecting their magma sources. We have thus far examined basaltic lavas from the Mascota volcanic field and Volcan San Juan to the west; calc-alkaline lavas from monogenetic centers of the Michoacan- Guanajuato and Chichinautzin volcanic fields, and the Palma Sola volcanic field in the easternmost TMVB. In general, B, Be, and Li systematics for basalts in other TMVB centers are similar to those at Colima: Be is higher in alkaline lavas than calc-alkaline lavas, while B contents are low relative to other arcs. All the lavas show elevated Li/Y and Li/Yb, and K/Li-La/Yb trends indicate slab-derived alkali enrichments and amphibole crystallization. However, along-arc changes are evident in mean B/Be (~7 in west TMVB centers to ~20 in the east) and Li/Yb (~11 in the west, ~6 in the east). Be/Nd ratios are essentially constant along the TMVB, but mean Be/Ti declines from ~1.3 in the west to ~0.5 in the east, implying increases in the mean extent of partial melting from W to E. Be/Ti varies inversely with the age of the downgoing plate, and positively with inferred slab temperatures (Curry et al, 2002), so it is possible that deeper transport of slab fluids to arc depths in E-TMVB leads to higher degrees of mantle melting, an inference consistent with B/Be variations. That Li/Yb and Be/Ti correlate positively may mean that the slab flux of Li changes minimally along the arc and changing melt extents dominate the signature; or that the role of amphibole is greater in W-TMVB than in the east, due perhaps to greater magmatic water contents (Kelley et al, 2007).