Experimental investigations were performed on the role molar volume stresses play during thin-film solid-solid reactions and phase transformations. Reactions and transformations in the thin-film system MgO-GeO2 were studied under two different conditions, viz. a planar thin-film geometry on a rigid substrate involving stress, and a microbeam geometry ensuring the immediate relaxation of any stresses via microbeam bending. Induced by reactions or transformations, respectively, the formation of the different phases (MgGeO3, Mg2GeO4, Mg28Ge10O48) and their polymorphs (clino- and orthopyroxene, spinel and olivine) was investigated by RBS, XRD, TEM/SAED and EDX as well as by in situ-TEM-observations applying a double-tilt heating holder. The results show that phase formation under thin-film conditions involving stress indeed differs from that under stress-free conditions. Stability ranges and transformation temperatures were distinctly shifted under conditions involving stress. Moreover, the crystallographic orientation of the olivine phase was stress-dependent, The observations are discussed in terms of different phase transformation mechanisms and a contribution of the stress enthalpy to the Gibbs free energy.