A pneumatic oxygen conserving device for supplying oxygen to a user via a single-tube cannula is described. Inhalation is sensed by negative pressure in an output line 11 which causes a sensing valve 21 to open, thus venting a main control volume 14 via vent line 22. This fall in pressure causes a main control valve 2 to open to supply a pulse of gas. This pressurizes the output line 11 which in turn causes the sensing valve 21 to close, thus preventing further venting. Volume 14 now re-pressurizes via line 15 and restrictor 16 and eventually reaches a level at which valve 2 closes and terminates the pulse of gas supplied to the user. A sensing delay valve 35 is incorporated in the vent line 22 from volume 14 to prevent the volume from venting after the first pulse has been delivered, until after the user has finished inhaling.