We present ab initio estimates of the gas-phase acidities Delta H-acid of the second-row metal hydrides NaH, MgH2, and AlH3. Our estimates are based on frozen-core CCSD(T) calculations using augmented triple- and quadruple-zeta correlation-consistent basis sets that include core polarization functions; these single-point calculations are extrapolated to the complete basis set limit and then adjusted to incorporate core-valence correlation effects, vibrational zero-point energy contributions, and thermal corrections. The final Delta H-acid values we obtain at T = 298 K are 345.26 kcal/mol for NaH, 363.83 kcal/mol for MgH2, and 372.93 kcal/mol for AlH3; the estimated uncertainty in these values is +/-0.8 kcal/mol. Our results suggest that the current NIST-JANAF recommended bond dissociation energy for NaH is too high by roughly 2 kcal/mol.