The nature of the sulfur deficit in pyrite, as found before by chemical analysis and density measurements, has been investigated by powder x-ray diffraction (XRD). Structure-factor refinements showed that the sulfur population parameter deviates several percent from unity, although the cubic-unit-cell edge a only varies in the order of 10-3 Å. To do these high-precision a measurements, a determination of the Cu Kα2 wavelength is made. Vegard's rule for sulfur-deficient pyrite is presented. The sulfur deficit is explained as a simple Schottky defect, whose vacancy-formation energy is about 0.3 eV. On the basis of ligand-field- and molecular-orbital-theory considerations, the band-structure model of pyrite is modified due to the presence of S vacancies. The most important electronic effect is the formation of defect states in the forbidden zone. This result will be discussed with respect to contradictory measurements concerning the electronic properties of pyrite.