The effect of local electric held at the NaCl(100) surface on the vibrational spectrum of adsorbed (vs free) CO is calculated and compared with previous experiments. We consider the change in both the vibrational frequency nu and the integrated IR absorption S. The calculation is based on the known molecular properties of free CO and uses S at F = 0 as calculated earlier by Poppe, Wijers, and van Silfhout [Phys. Rev. B 44, 7917 (1991)]. We show how to account for the position dependence of F along the molecular axis in calculating nu and S. This is important for the NaCl(100) surface since F varies strongly in the empty site where the CO adsorbs. The ratio of calculated to observed Delta nu in the Limit of low coverage is 0.98 +/- 0.17; the ratio of calculated to observed S with monolayer CO coverage is 1.10 +/- 0.12. The calculated effects agree with the data for CO on NaCl(100). We also discuss how the vibrational Stark effect may apply to CO on MgO(100) and at cation sites in zeolites.