An ionophore-doped sensing membrane phosphate (PO4) microsensor based on bis(dibromophenylstannyl)methane (Bis microsensor) is described. The Bis microsensor showed a Nernstian response. The response of the Bis microsensor was log-linear down to a monohydrogen phosphate ion (HPO4 2-) concentration of 0.5 μM (corresponding to 1.0 μM of orthophosphate at pH 7.2), whereas the detection limit of PO4-microsensors based on trialkyl/aryltin chloride was 50 μM of HPO4 2-. The Bis microsensor showed excellent selectivity for HPO4 2-against nitrite, nitrate, chloride, bicarbonate and sulfate, as compared with PO4 microsensors based on trialkyl/aryltin chloride. Dissolved oxygen, which is known to interfere with the response of a previously developed cobalt-based potentiometric solid-state PO4 microsensor, had no effect on the response of the ionophore-doped sensing membrane-type microsensors described herein. Only OH -(i.e., pH) interfered with the ionophore-doped sensing membrane-type microsensors.