This paper describes the use of a high velocity extractive sampling probe in a gas turbine fuel injector operating at 9 atm. This instrument has the ability to measure the temporal and spatial fluctuations of the mixture fraction. Knowledge of the fuel-air mixing characteristics is necessary to further decrease levels of pollutant emissions such as nitrogen oxides (NO x), and to sustain stable combustion. The extractive probe resolves temporal fluctuations due to the high flow rate through the sampling probe. The residence time of the sample gas in the sampling probe is sufficiently short so that axial diffusion on the relevant time scale can be ignored. Measurements were taken at two operating points: one at a stable low-emissions condition, and one at a condition where pressure oscillations in the combustor were high. At the second operating point, it was found that the frequencies of the pressure oscillations coincided with the frequencies of fluctuation in air-fuel ratio resolved by the probe.