The precision and accuracy of diffraction peak positions resolved from the powder X-ray diffraction spectra of cotton fibers by means of the residuals peak-fitting procedure of a commercial peak-fitting software package (PeakFit(R)) was investigated to explore the potential of such programs in providing reliable data that are not readily apparent in diffuse X-ray spectra. Each intensity spectrum was fitted by employing a Gaussian function as the peak profile. The precision of 15 pairs of the resolved diffraction peaks is 0.0017 mm; 9 of the 15 pairs fall into the 99% confidence interval. The precision in 20 of four commonly identified peaks through four independent fittings is from 0.002 to 0.014degrees. The 2theta accuracy of the resolved peaks is good in comparison to data calculated for both the traditionally accepted and the more recently revised concepts of the cellulose I crystal structure. A peak resolved at about 15.3degrees may be attributed to the triclinic crystal structure component of cellulose I. Although distinct peaks are not readily apparent in the diffuse spectrum of cellulose, peak-fitting procedures may provide additional data for structure determination. (C) 2004 Wiley Periodicals, Inc.