The chemical pressure control in (Sr2 - xCax)FeMoO6 (0 <= x <= 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr2-xCax)FeMoO6 samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe2+ + Mo6+)/(Fe3+ + Mo5+) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations. (c) 2007 Elsevier B.V. All rights reserved.