Abstract Annealing effects of FeSe 1− x Te x (0.6 ≦ x ≦ 1) single crystals have been investigated from measurements of the powder X-ray diffraction and specific heat. Through the annealing, several peaks of powder X-ray diffraction have become sharp and a clean jump of the specific-heat at the superconducting (SC) transition temperature, T c, has been observed for x = 0.6–0.9, indicating bulk superconductivity. For annealed single-crystals of x = 0.6–0.8, the SC condensation energy, U 0, and the SC gap, Δ 0, at 0 K have been estimated as ∼1.8 J/mol and 2.3–2.5 meV, respectively. The value of 2 Δ 0/ k B T c is 3.9–4.5, indicating a little strong-coupling superconductivity. Both the electronic specific-heat coefficient in the normal state, γ n , and the residual electronic specific-heat coefficient in the SC state, γ 0, have been found to show significant x dependence. The values of γ n are much larger than those estimated from the band calculation.