Thallium was introduced into a-SiC:H up to about 1% using the co-sputtering method. The dependence of the optical, electrical and optoelectronic properties on the impurity content, z, was investigated. In the range z < 10(-3), distinctive photoconductive effects appear and the optical bandgap is almost unchanged. The thermoelectric power data show that the type of conduction shifts from n-type to intrinsic with increasing z, which implies that Tl impurities act as dopants. Successful doping, which is difficult from the gaseous phase, was carried out. In the range of z above 10(-3), the films begin to exhibit alloying behaviour, such as a decrease in the optical bandgap, an increase in the dark conductivity and quenching of the photoconductive effect.