A method of operating a DNP system comprising: a cryostat (1); a magnetic field generator (2) located in the cryostat for generating a magnetic field in a working volume; a microwave cavity (8) within which the working volume is located; and a waveguide (18) for supplying microwave power to the microwave cavity; the method comprising: a) locating a sample in the working volume and subjecting the sample to a magnetic field generated by the magnetic field generator; b) supplying liquid coolant to the working volume to cool the sample; c) irradiating the sample with microwave power so as to hyperpolarize nuclear spins in the sample; and d) warming the sample following the irradiation step by expelling liquid coolant from the working volume while leaving the sample in the working volume.