We report on measurements of third sound in 3He-4He mixture films. The third sound velocity is measured as a function of temperature for 4He thicknesses between 3.6 and 5.8 atomic layers (a.l.) and 3He coverages up to 1.25 a.l. An anomaly in the third sound velocity is found around 200 mK for films with a 4He thickness of less than 5 a.l. and 3He coverages around 0.5 a.l. Comparison of the results is made with the model of Guyer and Miller and the calculations of Pavloff and Treiner [1, 2].