The transfer of liquid oxygen (LOX) from a storage vessel to a rocket engine generally requires the use of a pressurizing gas like nitrogen at a typical pressure of 3000 psi. The condensation of gaseous nitrogen to the liquid (LN2), combined with high miscibility, results in contamination of the LOX, which has a deleterious effect on engine performance. It has been proposed to monitor the LOX contamination through sound speed measurements. The purpose of this study is to measure the sound speed of LOX‐LN2mixtures under controlled conditions and to utilize the results in a practical LOX contamination monitor. The measurements, performed by the pulse‐echo technique on several LOX‐LN2compositions, reveal a slight deviation from a linear dependence of the sound speed upon the mole fraction of LN2. The results for pure LOX and pure LN2at − 195.8 °C and 1 atm pressure are 1008.5 ± 0.25% and 852.8 ± 0.32% m/s, respectively, and these are compared to measurements reported in the past.