The influence of different substrates on the ion concentration distributions in the large volume slot antenna microwave plasma source SLAN II was examined. The plasma chamber of SLAN II is formed by a quartz tube of 66 cm in diameter and 44 cm in length. Microwave power of 4 kW at 2.45 GHz is fed through 30 resonant coupling slots from an annular cavity. Ion concentration and electron temperature distributions were measured by use of a double Langmuir probe. The pressures of both argon and oxygen were changed from 1 Pa to 5 Pa. Axial ion concentration distributions were measured with and without circular substrates of 15 cm diameter, made of different materials (aluminum, stainless steel, glass) and placed in the center of the source perpendicular to its axis. The ion concentration at a position 2 mm from from the substrate surface is reduced by a factor of 2. in comparison with the plasma without substrate. Substrates cause a slight increase of electron temperature. The influence of the substrate on the ion concentration distribution is enhanced with increasing pressure and is stronger for argon than for oxygen. Only slight differences between plasma parameter distributions for different substrate materials are observed. The influence of axially positioned substrates on radial ion concentration distributions was also examined. In this case a weaker influence of the substrate on the ion concentration distribution is observed because of the vicinity of the direct power coupling zone (quartz tube).