Asphaltenes separated from two different North Sea oils were studied upon addition of two kinds of naphthenic acids by employing PFG-SE NMR (pulsed field gradient-spin echo nuclear magnetic resonance) and NIR (near-infrared) spectroscopy. The results indicated that there were interactions between asphaltenes and naphthenic acids. It also appeared as if the tendency of the acids to interact with asphaltenes depended on the asphaltene type. Furthermore, a concentration series of one of the asphaltenes was prepared, and a dramatic decrease in diffusion coefficients upon increased concentration was observed. This suggests that the asphaltenes studied were prone to self-associate in toluene-d(8).