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In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of [H.sub.2]O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass ([[delta].sup.11]B = +5.0 [+ or -] 0.9% and + 5.4 [+ or -] 0.5 [per thousand], respectively); (2) SIMS spot-analyses on [H.sub.2]O-poor obsidian (~0.15 wt% [H.sub.2]O) from younger Glass Mountain Dome YA (average [[delta].sup.11]B = + 5.2 [+ or -] 1.0 [per thousand]) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters ([H.sub.2]O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average [[delta].sup.11]B = + 3.2 [+ or -] 0.8 [per thousand], MSWD=0.4). Insignificant variations in [[delta].sup.B] between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor [sup.11]B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower [[delta].sup.11]B values (average [[delta].sup.11]B = + 2.2 [+ or -] 0.8 [per thousand] and -0.4 [+ or -] 1.0 [per thousand]) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; [[delta].sup.11]B = + 5.7 [+ or -] 0.8 [per thousand]). These variations in [[delta].sup.11]B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of [[delta].sup.11]B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.