A set of general guidelines for structure re®nement using the Rietveld (whole-pro®le) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld re®nement are discussed with a view to guiding newcomers in the ®eld. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features speci®c to data from neutron (both constant-wavelength and time-of-¯ight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) re®nement of pro®le parameters, (v) Fourier analysis with powder diffraction data, (vi) re®nement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.

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Dedicated to Professor Hans Paulsen on the occasion of his 75th birthday (23.XII. 1996) The solid-state structures of three stereoisomers, 1-3, of the cyclic tetramer of 3-aminobutanoic acid are presented. These cyclo-p-peptides were found to be highly insoluble materials, and it proved to be impossible to grow crystals of suficient quality for X-ray single-crystal analysis. The samples of 1-3 were, however, suitable candidates for structure determination from powder diffraction data (Fig. Z), and the application of this method is described. All three isomers have been found to adopt tubular structures (Figs. 2-4) in a fashion similar to those already observed for certain cyclo-a-peptides. The stacks of 16-membered rings are held together by four nonlinear C=O ...H-N H-bonds between pairs of molecules (Fig. 5 ) .