Action spectroscopy of gaseous Me3BF- and Me2BCH2-, determined using IR multiple photon dissociation (IRMPD), exhibits experimental peak positions in good agreement with those predicted by anharmonic DFT calculations at the B3LYP/aug-cc-pVTZ level. Comparing spring constants among boron-carbon bonds and consideration of isoelectronic systems reveals that the anions have B-CH3 bonds a factor of 1.3-1.4 weaker than those in neutral trimethylboron, suggesting a greater role for hyperconjugation in 8(CH3)(3) than inferred from previous vibrational spectroscopic studies.