A detailed analytical model has been developed to simulate isotope-releasecurves from thin-foils ISOL targets. It involves the separate modeling of diffusion and effusion inside the target.The former has been modeled using both first and second Fick'slaw.The latter,effusion from the surface of the target material to the end of the ionizer, was simulated with the Monte Carlo code Mol Flowþ. The calculated delay-time distribution for this process was then fitted using a double-exponential function. There lease curve obtained from the convolution of diffusion and effusion shows good agreement with experimental data from two different target geometries used at ISOLDE. Moreover, the experimental yields are well re- produced when combining there lease fraction with calculated in-target production