Formation of pyrite (FeS2) thin films by sulphuration of metallic layers (Fe) and bilayers (Fe-Ti) has been investigated. An experimental technique, which allows the measurement of both the electrical resistance (R) and the thermoelectric coefficient (S) of the films during the sulphuration process (in situ), has been used. Sulphurated Fe-Ti bilayers appear to be n-type semiconductors in contrast with sulphurated Fe films, which are always p-type semiconductors. The change of conductivity type has been attributed to the doping effect of FeS2 films by Ti. (c) 2004 Elsevier B.V. All rights reserved.