[en] The electronic specific heat of as-grown and annealed single-crystals of FeSe_1-xTe_x (0.6 ≤ x ≤ 1) has been investigated. It has been found that annealed single-crystals with x = 0.6-0.9 exhibit bulk superconductivity with a clear specific-heat jump at the superconducting (SC) transition temperature, T_c. Both 2Δ₀/k_BT_c [Δ₀: the SC gap at 0 K estimated using the single-band BCS s-wave model] and ΔC/(γ_n - γ₀)T_c [ΔC: the specific-heat jump at T_c, γ_n: the electronic specific-heat coefficient in the normal state, γ₀: the residual electronic specific-heat coefficient at 0 K in the SC state] are largest in the well-annealed single-crystal with x = 0.7, i.e., 4.29 and 2.76, respectively, indicating that the superconductivity is of the strong coupling. The thermodynamic critical field has also been estimated. γ_n has been found to be one order of magnitude larger than those estimated from the band calculations and increases with increasing x at x = 0.6-0.9, which is surmised to be due to the increase in the electronic effective mass, namely, the enhancement of the electron correlation. It has been found that there remains a finite value of γ₀ in the SC state even in the well-annealed single-crystals with x = 0.8-0.9, suggesting an inhomogeneous electronic state in real space and/or momentum space. (author)