The microstructure and the thermoelectric properties were systematically determined in the Fe2V1+xAl1-x, Fe2+xVAl1-x, Fe2-xV1+xAl series to investigate the influence of self-substitution on the Fe2VAl Heusler alloy. In the explored range of compositions (−0.1 < x < 0.1), all these series are solid solutions, which form anti-site defects to accommodate the off stoichiometry. They all crystallize in the cubic L21 structure, but their lattice parameter unusually increases with lxl. A Bader analysis based on Density Functional Theory calculations indicates that these uncommon lattice parameter changes arise from variations in the interatomic electron transfer. The antisite defects behave like dopants that control the conduction type and charge carrier concentration. This leads to large thermoelectric power factor (PF) in the Fe2V1+xAl1-x series, which displays the largest electronic mobility. PF = 6.7 mW m−1 K−2 at 250 K and PF = 3.2 mW m−1 K−2 at 325 K are reached in n-type Fe2V1.03Al0.97 and p-type Fe2V0.985Al1.015 respectively. The lattice thermal conductivity systematically decreases upon self-substitution, but with differences among the series which can be traced back to the interatomic electron transfer unveiled by the Bader analysis. Finally, the figure of merit is improved to ZT = 0.06 at 500 K in p-type Fe2V0.93Al1.07 and ZT = 0.15 at 420 K in n-type Fe2V1.08Al0.92.