Isodimorphism for two Aurivillius phases BaBi4Ti4O15 and CaBi4Ti4O15 with different symmetry was studied. Synthesis techniques have been optimized and solid solution with the composition Ba1-xCaxBi4Ti4O15 (x - molar part of the component, x = 0, 0.25, 0.5, 0.75, 1) have been prepared. The structure of the compounds has been studied using X-ray diffraction and Raman spectroscopy. It was found that solid solutions with tetragonal and orthorhombic symmetry were formed in the range of 0 < x < 0.75 and at x >= 0.75, respectively. The basic laws of thermal expansion for the binary system BaBi4Ti4O15-CaBi4Ti4O15 have been established.