Abstract: A two-stage pump (9) for a continuous ink jet printing system comprises a first stage (A) which includes a chamber (91,92) having an inlet (93) and an outlet (94) via a non-return valve (115) to a channel (95). The first stage chamber (91,92) is divided by a dished diaphragm (101) movable by a first cylinder (97) mounted on a pump shaft (98). The pump (9) has a second stage (B) comprising a chamber (104,105) having an inlet (106) from the channel (95) and an outlet (107). The second stage chamber (104,105) is divided by a rolling diaphragm (116) movable by a second cylinder (108) mounted on a pump shaft (98). The cylinders (97,108) are arranged to be driven, and thus drive their respective diaphragms (101,116), 180.degree. out of phase so that ink and air enters the first stage chamber (91,92), is pressurised by the first cylinder (97) and passed through the channel (95) to the second stage chamber, and is further pressurised and passed out of the outlet (107) of the second stage chamber (104,105).