The influence of a thin metal underlayer on the chalcopyrite growth was studied. CuInSe2 (CIS) and CuIn1-xAlxSe2 (CIAS) thin films were grown by a two-stage process (stacked metal layers evaporation and subsequent selenisation) onto Mo and bared soda-lime glass substrates. Due to the difficulty found for get CIAS samples onto Mo, a thin Al layer was evaporated before the metallic stack destined to obtain CIS and CIAS films. This work shows the differences that appear between the CIS and CIAS samples evaporated onto both substrates, with and without the thin Al underlayer. By means of the characterisation of the samples, it has been observed that the use of Mo contact modifies the usual formation pathway of CIAS for the two-stage process. The evaporation of the thin Al layer on Mo promotes a higher grade of reaction of the CIAS phases.