Hydrogen trapping and desorption in a martensitic steel with mixed (Ti,Mo)C precipitates have been investigated by means of Thermal Desorption Spectrometry (TDS) analysis. Results indicated that the spherical (Ti,Mo)C precipitates of 36-60 nm could not absorb hydrogen during electrochemical charging, while the (Ti,Mo)C precipitates of 1-5 nm were effective hydrogen trapping sites. In spite of a relatively low desorption activation energy of 16.4 kJ/mol, which is far lower than that from pure coherent TiC, the hydrogen trapped by the fine (Ti,Mo)C carbides could not desorb during atmospheric exposure.