[en] We present the results from structural analysis of single crystalline FeSe_1−xTe_x and magnetisation measurements of as-grown FeSe_0.35Te_0.65 samples and after they have been annealed in air. Raman spectroscopy revealed that air annealing leads to the formation of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) on the surface of FeSe_1−xTe_x, with the bulk beneath being free of iron oxide. Furthermore, we observe an increase in the superconducting transition temperature, T _c, from 7 to 14 K, and in the critical current density, J _c, by one order of magnitude for all applied magnetic fields. A comparison of two FeSe_0.35Te_0.65 samples of different thickness indicates a possible size dependence for the optimal annealing conditions. Annealing for a second time (prolonged annealing) was found to induce a surface barrier, indicated by the transition from a symmetric to a highly asymmetric magnetic hysteresis loop. (paper)