Method for the dynamic nuclear polarisation (DNP) of a compound comprising one or more carboxyl groups, said method comprising a step of adding a radical of formula (I) wherein M represents hydrogen or one equivalent of a cation and Rl which is the same or different represents a group -(CH2)n-X-R2, wherein n is 1, 2 or 3 X is O or S and R2 is a straight chain or branched C1-C4- alkyl group is used as the paramagnetic agent in said DNP process.