The influence of nitrogen cluster states (CS) on the conduction band (CB) structure of GaAs 1-x N x is probed by measuring the effective mass and gyromagnetic factor of carriers for a wide range of nitrogen concentrations (from x< 0.01% to x= 0.7%). An unusual compositional dependence of these two important CB parameters is found. This behaviour is well reproduced by a modified k· p model taking into account the nonmonotonic loss of Gamma character of the CB minimum due to multiple crossings between the red-shifting ...