[en] The electronic properties of unstrained unrelaxed InAs_xSb_1_−_x alloys have been determined in a wide range of alloy compositions using IR magnetospectroscopy, magnetotransport and IR photoluminescence. All studied alloys have n-type background doping with electron concentration decreasing with the Sb content. The composition dependence of the background doping concentration follows an empirical exponential law in a wide range of compositions. Both bandgap and electron effective mass dependence on alloy composition exhibit negative bowing reaching lowest values at x  =  0.63: E _g  =  0.10 eV, m*  =  0.0082 m _0 at 4.2 K. The bowing coefficient of 0.038 m _0 obtained for the electron effective mass is in good agreement with that obtained from the Kane model. (paper)