Abstract: A liquid chromatograph includes a multi piston reciprocating pump for delivering liquid at a desired flow rate to a separating column. The pistons in the pump have overlapping delivery strokes and when the pump is provided with two pistons their rate of advance is halved during the overlapping delivery periods. The instant when both pistons start delivery is dependent on solvent compressibility and the delivery pressure of the pump and thus will vary according to the operating conditions. The delivery pressure of the pump is monitored and if a pressure increase (or decrease) is found during one pump cycle between the start of stroke of one piston (SST1) and the end of delivery of the other piston (EOD2) the instant when the speed of advance of the pistons is halved (HS1) is advanced (or retarded) in the next pump cycle.