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Crystallographic and magnetic properties of bulk Co2Fe(Ge1–xSix) alloys with 0≤x≤1, synthesized by arc melting method, have been studied. Co2FeSi alloy has been found to crystallize with L21 structure, but the super-lattice peaks are absent in the X-ray diffraction patterns of alloys containing high Ge concentration. Unit cell volume of this series of alloys decreased from 185.2 to 178.5 Å3 as Si content was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (TC). TC showed a systematic variation with x. A comparison between the values of saturation magnetization (Ms) and effective moment per magnetic atom pc estimated from the temperature dependent susceptibility data above TC, shows that the alloys have half-metallic character. The alloy with x=0 follows Slater-Pauling (S-P) rule with Ms of 5.99µB. However, Ms for the alloy with x=1.00 was found to be 5.42µB, which is lower than the value of 6.0µB predicted by S-P rule. Since atomic disorder is known to affect the Ms and electronic structure of these alloys, ab initio calculations were carried out to explain the deviation in observed Ms from S-P rule prediction and the half-metallic character of the alloys. Ab initio calculations reveal that alloys with L21 structure have Ms value as predicted by S-P rule. However, introduction of 12.5% DO3 disorder, which occurs due to swapping of Co and Fe atoms in the unit cell, decreases Ms of alloys with x>0 from the S-P prediction to values obtained experimentally. The results analyzed from the view point of electronic structure of the alloys in different ordered states bring out the influence of disorder on the observed magnetic properties of these technologically important alloys. [ABSTRACT FROM AUTHOR]