It has been discovered that facile glycation of proteins can be achieved by colyophilization of a protein with a reducing sugar, subjecting the lyophilized mixture to a vacuum (10 to 50 millitor) and incubating at an elevated temperature (50 to 100.degree. C.) for 1 to 24 h. A stable ketoamine derivative is formed with amino groups in the protein and no advanced glycation end products (browning reaction) are observed, as is the case with aqueous glycation procedures. Another novel feature is that the in vacuo glycation reaction takes place with the protonated amine and not the deprotonated amine as is believed to be the case for aqueous glycation reactions. Advantage can be taken of the in vacuo glycation reaction to achieve facile covalent cross-linking of proteins by lyophilizing protein or proteins with compounds containing two or more reducing sugars separated by a linker.