The partial pressure of vibrationally excited hydrogen has been measured in a flowing gas using the technique of vacuum ultraviolet laser-induced fluorescence. In most of the studies H2 (v=1) was generated with a microwave discharge. For this source we measured in detail the effects of the partial pressures of H2 and Ar, the flow rate, the microwave power, and the construction of the flow tube. A more limited study was also performed using a thermally heated flow tube.