This paper presents experimental data on solid-phase synthesis in double-layer thin-film systems. The rule of the first phase formed at the interface of film reagents at elevated temperatures of annealing is formulated for solid-phase reactions determined by martensite transformations. The temperature at which synthesis is initiated in NinTi, CdnAu, and AlnNi films coincides with the temperature of the back martensite transition in NiTi, AuCd, and AlNi alloys so that martensite phases are formed in the reaction products. The first phase rule was also verified for solid-phase synthesis in CdnAg, NinMn, FeMn, and AunMn systems. In thin films, low-energy processes initiate solid-phase reactions associated with martensite transformations, mass transfer of reagents being up to 200 nm. The martensite model of atomic transfer through the reaction product during solid-phase synthesis in thin films is considered. Martensite shifts can play the dominant role in mass transfer of reagents through the reaction product. It is assumed that the first phase initially formed at the interface of film reagents is formed irrespective of the mode of solidphase synthesis initiation.