A C-13 nuclear magnetic resonance study (with cross polarization and magic angle spinning) is presented of commercially available cellulose acetate and cellulose propionate as powder samples and also in different membranes. Some physical properties of the films are interpreted in the light of data obtained for the principal components of the shielding tenser of C-13 nuclei on carbonyl groups. In particular, it is shown that the shielding asymmetry factor, measured on samples with a regular shape and with a fixed orientation in the rotor, is a sensitive probe to ordered domains in the polymers.