We present a detailed study of the models for diffusion controlled bimolecular reactions in phospholipid membranes analyzing the case of the photodimerization of long lived (200 ns) and short lived (10 ns) excited species under steady state irradiation. It is shown that only when using the 2D formalism for the diffusion-controlled rate constant, k(diff), do the parameters retrieved have physical meaning. In certain cases the use of a 3D law instead of the 2D specific law does not lead to significantly different results. However, this agreement between the two models is misleading and depends on the specific system, namely on the way the 3D concentration is calculated and on the position of the reactant relative to the center of the bilayer. The models were tested for the photodimerization of 6-(9-anthroyloxy)stearic acid (6-AS) in a membrane model system of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The quenching of the excited state is diffusion controlled but only a fraction of quenching, gamma, leads to dimerization. Since we know from independent measurements that gamma must vary between 0.05 and 0.1 we may conclude that the value of gamma = 0.3 obtained with the 3D law without considering the time dependence of k(diff) is out of range, while gamma approximate to 0.1 obtained for both 2D and 3D complete laws is acceptable. We further discuss the physical meaning of the application of the 3D law to this system and the method used for the calculation of the 3D concentration. The lateral diffusion coefficient of the reactants, necessary for the calculations, was independently measured in a model system using the technique of fluorescence recovery after photobleaching.