The transient molecule HOPO has been detected in the gas phase for the first time using high resolution infrared laser absorption spectroscopy. The molecule is formed in the reaction between the products of a microwave discharge in hydrogen and oxygen, and white phosphorus vapour. Many absorption lines were detected in the region of the predicted P-O (upsilon(4)) and P=O (upsilon(2)) stretching modes of cis-HOPO around 850 and 1250 cm(-1) respectively. About 170 rotational lines of the 850 cm(-1) spectrum have been assigned to an a-type transition of an asymmetric rotor. The derived rotational constants are compatible with the values for cis-HOPO produced by a density functional theory calculation.