Pulse field gradient-NMR (PFG-NMR) spectroscopy is determined to be a more suitable method for the investigation of self-association processes in multi-component (co)polymer systems than light scattering methods. Here the co-micellization of mixtures of the diblock copolymer polystyrene-block-(hydrogenated polyisoprene) (PS-HPI) and the triblock copolymer polystyrene-block-(hydrogenated polybutadiene)-block-polystyrene (PS-HPB-PS) in decane is investigated by PFG-NMR spectroscopy and the results compared to those experimentally determined by static (SLS) and dynamic (DLS) light scattering. As expected, diffusion coefficients determined by PFG-NMR spectroscopy are systematically lower than those from DLS. The PFG-NMR of measurements provided higher values c(tr)(e)(X)/c(tot) than the model calculations, illustrating that the basic assumption used in the calculations, i.e., that the number concentration of co-micelles in mixed solutions follows the dilution with a triblock copolymer solution, 1 - X, is not fully valid at high X (weight fraction of PS-HPB) values.