X-ray diffraction studies on Co2Fe(Ga1-xSi5) alloys did not show the characteristic L2(1) super-lattice peaks for alloys with x <= 0.5. The saturation magnetization (M-s) of alloy compositions with x = 0.75 and x = 1.00 deviated from the values predicted by Slater-Pauling (S-P) rule. In order to understand this deviation in M-s and its implication on the half-metallic character of these alloys, a systematic ab initio study was carried out using generalized gradient approximation (GGA) and GGA with on-site Coulomb interaction (GGA + U). Total magnetic moment obtained from the GGA calculation agrees well with S-P rule and experimental results obtained for alloys with high Ga concentration (i.e. for x = 0 and 0.25) and gets overestimated by the inclusion of U. However, the value predicted by the S-P rule is obtained for the alloys with x = 0.50, 0.75 and 1.00, only after the inclusion of U. Similar behavior was observed in the appearance of the energy gap in density of states of the spin-down band and hence the half-metallic nature of the alloys. The deviation in measured M-s of the alloy with x = 1.00 is attributed to the presence of small amount of DO3 disorder in the alloy as revealed by the ab initio studies. Comparison of experimental and theoretically estimated properties of Co2Fe(Ga1-xSix) alloys provides insight to the variation in electron-electron interaction and the influence on atomic site disorder in these alloys on their half-metallic character. (C) 2016 Elsevier B.V. All rights reserved.