The present invention provides a novel process for preparing a cell-binding agent cytotoxic agent conjugate. The process comprises the steps of: (a) reacting a cytotoxic agent with a bifunctional crosslinking reagent represented by the structural formula(I) or a salt thereof, in a buffer solution comprising a buffering agent to provide a first mixture comprising a cytotoxic agent-linker compound, wherein the buffer solution has high buffer capacity； and (b) reacting the first mixture comprising the cytotoxic agent-linker compound from step (a) with a cell-binding agent in a solution having a pH of 4 to 9 to provide a second mixture comprising the cell-binding agent cytotoxic agent conjugate. The cell-binding agent cytotoxic agent conjugates prepared according to the processes described herein are also included in the present invention.
