Three different mechanisms for disilane (Si2H6) chemisorption on Si(100) are discussed, and experimental evidence is presented that is consistent with occurrence of all three under different conditions. Direct formation of chemisorbed SiH2, in competition with formation of chemisorbed SiH3, is proposed on the clean surface. Direct recoiling results support chemisorption on H-terminated surfaces via insertion into SiH bonds at surface temperatures (Ts)=350°C and above. Estimates of enthalpy changes for each mechanism are given, based on the limited available bond-energy data. © 1991.