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Summary:The desorption from slightly soluble spread monolayers at the air/water interface is studied by compressing the monolayer suddenly, whereafter the surface tension is followed as a function of time. As far as the deviation from equilibrium is small a relation is obtained giving the transient surface tensions with time. This relation is obtained from the modulus of elasticity by inverse Fourier transformation. Two relaxation mechanisms are presented, in one the transfer from the monolayer to the subsurface is the rate limiting step (barrier), in the other the diffusion from the subsurface to the bulk is the rate limiting step (diffusion controlled). It was found that for lauric acid, myristic acid, the bile acids and 1-12 didodecanoic acid, the desorption process is diffusion controlled and good agreement between theory and experiment is obtained.Abstract: An equation is presented to give the dynamic surface tension with time for diffusion controlled adsorption/desorption processes. Principally, this equation is only valid for small deviations from the equilibrium surface tensions. In practice however this restriction is not so severe. The theory is tested for desorption of slightly soluble spread monolayers of lauric and myristic acid, cholic, desoxycholic and lithocholic acid and for 1-12 dodecanoic acid. Excellent agreement is achieved.