WO3-doped ceramics with the composition Ca3Co4−xWxO9 and x=0, 0.02, 0.05 0.1 and 0.2 were prepared by the classic ceramic procedure from fine oxide powders and sintering in air at 900°C. The influence of the composition on the microstructure development and the thermoelectric characteristics in the temperature range from 40°C to 615°C was studied. The formation of a solid solution with the composition Ca3Co3.88W0.12O9 was determined and indicates a much lower limit of WO3 solubility in the Ca3Co4O9 than has been reported previously. Doping with WO3 resulted in more irregularly shaped, sharp-edged and thinner plate-like grains in comparison to the grains in the un-doped Ca3Co4O9 ceramics. The un-doped ceramics had an electrical conductivity σ of 31S/cm, a Seebeck coefficient S of 147μV/K and a thermal conductivity κ of 1.15W/mK, giving a figure of merit ZT of 0.06 at 615°C. The WO3-doping decreased the electrical conductivity σ to 18S/cm, increased the Seebeck coefficient to 157μV/K and also decreased the thermal conductivity κ to 0.88W/mK, resulting in an overall reduced ZT of 0.047 for the Ca3Co3.88W0.12O9 ceramics.