The near infrared reflectance spectroscopy (NIRS) method of analysis, as applied to cotton, offers a direct method to measure maturity of the fiber. We wish to report here a test of the hypothesis that the fundamental measurements of maturity (wall thickness, relative maturity and degree of thickening) are linearly related to the geometry of the fiber as assessed by NIRS analysis. Indeed, a linear function (probability, p < .0021) was found between the various measures of maturity by image analysis and the spectroscopic measurement of maturity. These associations were retained (p < .0317) in cotton that had been purified by solvent extraction; there was no evidence of the associations (p > .1854) in the chemical residues from the extractions. The relationships did not exists (p > .1968) with ballmilled cotton. By contrast, a similar hypothesis test with other geometrical properties (wall area, fiber diameter and fiber area) resulted in lack of conclusive evidence of a linear association with NIRS analysis. The physical characteristic of the fiber detected by NIRS analysis in the correlation with maturity is directly related to wall thickness. The chemical constituent in cotton detected by NIRS analysis in the correlation with maturity is cellulose, the chief component of the fiber wall.