A new servo-control divided-flow method system has been developed, capable of imposing and controlling relative humidity (RH) and hence total suction in soils with improved stability compared with previous similar approaches. Streams of dry and saturated air are mixed in appropriate proportions to control the required RH around soil samples within sealed desiccators. The novelty of the system lies in the operation of valves used to control these streams, which are regulated by microcontrollers continuously receiving and processing hygrometer RH measurements from within the desiccators, as well as in the proposed limits within which the technique should be operated, which have not been detailed in the past. More than one desiccator can be connected in parallel within the system, each controlled independently, allowing for multiple soil samples to be tested under different RH at any one time. Another major advantage is that accurate control of RH can be achieved even in the absence of strict ambient temperature control in typical laboratory environments, significantly simplifying previous designs. RH can be controlled with a stability of ±0.2% and with minimal relative uncertainty in suction from 10 to 80%. This relates to 282 to 26 MPa of total suction, allowing soil-water retention curves (SWRCs) to be generated at very high suctions, relevant to highly active clays used as buffers for nuclear waste storage. The new system’s efficiency and accuracy are demonstrated through comparison of: (i) RH measurements with known values relating to specific salt solutions; and (ii) an SWRC generated for Wyoming sodium bentonite with data from the literature. The versatility of the system is also demonstrated through its incorporation into a RH-controlled oedometer with relevant data presented and discussed