We have studied the structural and superconducting properties of La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] by neutron and x-ray powder diffraction, thermogravimetry, ac susceptibility, and dc magnetization. The kinetics of the synthesis reaction were studied in order to determine the optimum conditions for preparing single-phase samples. The room-temperature and low-temperature structural properties, as determined by neutron powder diffraction, are discussed in detail. By comparing the results of the neutron-diffraction and magnetic measurements, we conclude that superconductivity exists in both the orthorhombic and tetragonal phases. The thermodynamic conditions for the stability of the La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] phase are also investigated.