We describe methods that allow either carbodiimides or other carboxyl-reactive substances to be mixed with solutions of carboxylic acids or phosphates or amines or combinations thereof, so as to form a homogeneous mixture which is then dried, preferably in a freeze drying process. The mixture is then contacted with an entity, which preferably involves the dissolution of the mixture with a buffered solution of the entity, so as to initiate a conjugation reaction between the entity and a component in the mixture.