The invention provides processes for manufacturing cell-binding agent-cytotoxic agent conjugates of improved stability comprising performing the modification reaction at a high pH. The inventive processes comprise contacting a cell-binding agent with a bifunctional crosslinking reagent in a solution having a pH of 7.1 to 9 to covalently attach a linker to the cell-binding agent and thereby prepare a mixture comprising cell-binding agents having linkers bound thereto.
