The electron effective mass, m e, has been determined by magnetophotoluminescence in as-grown and hydrogenated Ga As 1− x N x samples for a wide range of nitrogen concentrations (from x< 0.01% to x= 1.78%). A modified k∙ p model, which takes into account hybridization effects between N cluster states and the conduction band edge, reproduces quantitatively the experimental m e values up to x⩽ 0.6%. Experimental and theoretical evidence is provided for the N complexes responsible for the nonmonotonic and initially puzzling compositional dependence of the electron mass.