The activation energy for thermal dissociation of hydrogen from silicon dangling‐bond defects (Pbcenters) has been measured using both (111)‐ and (100)‐oriented samples. The behavior of each of the threePbvarieties [P111bat the (111) interface,P100b0andP100b1at the (100) interface] is compared. ForP111b, excellent agreement with previous results by Brower [Phys. Rev. B42, 3444 (1990)] is obtained. The activation energies of the (100)‐interfacePbcenters are slightly higher, assuming the same vibrational frequency, and, unlike that ofP111b, are affected by a postoxidation anneal. ©1995 American Institute of Physics.