Carbon C-13 spin-lattice relaxation times, T1, of Serine, Threonine, Phosphoserine and Phosphothreonine have been measured in D2O at neutral pD. Changes in over-all motions under phosphorylation has been detected. Segmental motions are not affected by phosphorylation. With the aid of T1(C-13) values of other amino acids an empirical relationship between NT1 and molecular weights (Mw) is tested that seems valid also for small peptides. Such relationship allows to interpret results from T1 measurements with the aid of a simple spherical model.