Hydrogen (H)- and deuterium (D)-terminated Si (100) surfaces are investigated by thermal desorption spectroscopy (TDS). The H/D exchange reaction, using 1.6% DE creates partially D-terminated Si surfaces from passively H-terminated Si by using 1.6% or 49% HE The H/D exchange reaction rate, determined from TDS intensity changes, follows natural-logarithmic equations. The half-exchange time is 12.8 min. The 49% HF pretreated samples show rapid surface roughness changes in the initial stage of the 1.6% DF treatment; these changes are detected by rapid TDS H-2 intensity decreases. SiFx and F-2 peaks are detected by TDS. F/D exchange occurs on the 49% HF pretreated samples along with H/D exchange. The activation energy for deuterium termination is 8.7 kcal/mol.