Knudsen cells used for molecular beam deposition of organic materials are usually derived from the ones employed for conventional epitaxy of inorganic semiconductors. However, organic molecular materials need rather low evaporation temperatures and a very precise temperature control in the range from 100 to 400 degreesC. Therefore, a thermal model of the cell is needed, which, taking into account the peculiarity of organic molecular beam deposition, gives the thermal dynamics of the cell and can be used for developing accurate algorithms for thermal programming and control. In this article such a model is developed and fully assessed through a comparison with experimental results; on this basis, all information necessary fur precisely controlling and programming Knudsen cells for organics is obtained.