The present invention provides a process for the preparation of a 11 C of 18 F- labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX or R 1 X, wherein X represents a leaving group; R represents an alkyl or alkylene containing group; R 1 represents an alkyl or alkylene containing group which group comprises a 18 F-label; and said group R and/or the carbon dixoide has a 11 C-labelled carbon atom.