A set of 12 reservoir core samples has been studied by proton spin-lattice relaxation measurements. Pore-size distributions for these samples have been obtained using the isolated sphere model. The model is based upon the theory of Brownstein and Tarr for relaxation of a fluid within a pore. Measurements of the initial slope of the relaxation curves did not prove an effective means of determining the value of the surface relaxation strength for input to the model, so the effect of varying surface relaxation strength was investigated via a grid search. The study shows that, with the spherical model, a single value of the surface relaxation strength is not appropriate to describe the relaxation behaviour of all the samples. Further work is in progress to determine whether the identified variation in surface relaxation strength is genuine, or an artefact of the simple model of the rock structure.