Penetration of complexes into the resin layer can dramatically increase the contribution of complexes to the metal flux measured with a DGT (diffusive gradients in thin films) sensor, but equations to describe this phenomenon were not available. Here, simple approximate analytical expressions for the metal flux, the lability degree and the concentration profiles in a DGT experiment are reported. Together with the thickness of the reaction layer in the gel domain, the effective penetration distance into the resin layer that would be necessary for full dissociation of the complex (λML) plays a key role in determining the metal flux. An increase in the resin-layer thickness (r) effectively increases the metal flux and the lability degree until r ≈ 3λML. For the usual DGT configuration, where the thickness of the gel layer exceeds that of the resin layer, the complex is labile if r > (DML/kd)1/2, where DML is the diffusion coefficient of the metal complex and kd its dissociation rate constant. A general procedure for estimating the lability of any complex in a standard DGT configuration is provided.