A rotor for use in a centrifugal evaporator is described, having a plurality of sample holder receiving means and fitted with an automatic balancing device for reducing out of balance forces that arise from the uneven distribution of sample holders around the rotor as the rotor is rotated during an evaporation process. A process of evaporating a plurality of samples is also described, in which sample holders containing samples are loaded initially and centrifuged, the rotor is stopped and some of the sample holders are removed and/or other sample holders are added before the rotor is spun again; wherein a force to compensate for different and uneven loading of the rotor by the samples and sample holders is generated by a balancing device which comprises a housing defining an annular path which contains at least one member which is free to move therearound but is otherwise constrained to remain therewithin.