Surface tension measurements on solutions of octanoic acid in water at 2Q-OC are recorded. Data are also given on the variation of surface tension of octanoic acid solutions with pH at 20oc at ionic strength 0.01. The effect of temperature on the surface tension of solutions in 0.1 N NaOH and HCl was also studied. The temperature effects were used to calculate the heats of adsorption of the unionised and ionised fatty acid, and these heat calculations are used to test theories of the ionic double layer. The effect of temperature on the equation of state of the monolayers is compared with the theory of Davie
