The influence of nitrogen cluster states on the conduction band (CB) structure of GaAs 1-x N x is probed by measuring the effective mass and gyromagnetic ratio of electrons for x< 0.7%. An unusual compositional dependence of these two important CB parameters is found. Such behaviors are well reproduced by a modified k· p model taking into account a non-monotonic loss of γ character of the CB minimum due to multiple crossings between the red-shifting conduction band edge and N cluster states. As well, sudden variations in the ...