The temperature variation of the electrical resistivity ρ and the Seebeck coefficient S of Heusler-type Fe2VAl1-xSi x(0≤x≤1) alloys has been investigated. We have shown that the transport parameters are very sensitive to doping. For the x ≤ 0 sample, high values of ρ and negative temperature coefficient of resistivity (TCR) have been observed. As the Si concentration increases, ρ decreases and the TCR changes its sign, while S shows significant changes in magnitude as well as sign when Al is replaced with Si. These changes appear to be reminiscent of a metal to semiconductor transition. It has been shown that the conventional transport theories proposed for intermetallic alloys or semiconductors cannot explain the transport behaviour in the whole temperature range of the present study. Low-temperature resistivity data of x ≤ 0-0.02 samples could be described with a gapless semiconductor model. The strong composition dependence of S and ρ is attributed to the sharp variations in electronic density of states at the Fermi energy. It is also shown that by optimum doping one can achieve very large values of power factor (P). The estimated power factor at room temperature is observed to be highest (2.23 × 10-3 W mK-2) for x ≤ 0.06 and comparable to that of conventional thermoelectric material. At lower temperatures P is found to be even higher than that of conventional thermoelectric material. © 2005 IOP Publishing Ltd.