Tungsten carbide films have been deposited by low-pressure chemical vapour deposition from a WF6/H-C3(8)/H-2 mixture on Ta and Ni substrates. Single-phase WC films could be deposited on Ta in a broad vapour composition range at 900 degrees C. A mixture of WC and W2C was deposited in the temperature range 700-850 degrees C, while an amorphous film was obtained at 650 degrees C. The temperature behaviour suggests that deposition of carbon is a limiting factor in the growth process. The deposition process on Ta could be separated into two parts : a fast substrate reduction step of WF6 leading to the formation of metallic W followed by a slower formation and deposition of WC. The growth behaviour on Ta was also affected by tantalum carbides at the film-substrate interface. A different growth behaviour was observed on Ni. It was found that several eta-carbides (i.e. Ni2W4C and Ni6W6C) were formed during a fast initial growth stage. Later on, the eta-carbides reacted with carbon under the formation of WC and free Ni particles. It was also found that the carbon deposition rate on Ni substrates was higher than on Ta. This was explained by a catalytic process where Ni particles on the film surface favoured carbon deposition.