Mechanical and chemical disorders are investigated in GaAsN alloys with N content close to the solubility limit, similar to2-3 %. One attractive characteristic, yet unexplored, of N-based III-V ternary alloys is that they consist in mixed crystals with a sharp contrast in the bulk modulus of the two constituents. Raman scattering is well-suited for its investigation because it addresses the force constant of the bond, which depends on the mechanical properties of the host matrix. In the Ga-N region, we observe a mode at 425 cm(-1), i.e. below the local mode of N in GaAs at 470 cm(-1). This mode has degenerated LO and TO components and seems to grow at fixed frequency with N content. From a direct comparison with Zn-Be chalcogenides, which exhibit a similar mechanical contrast, it is attributed to GaN vibrations in hard-like N-rich bounded regions, dispersed within the soft-like Ga-rich matrix. In the GaAs region we use Si-doping as a sensitive probe to investigate the structural quality close to the solubility limit. At low N content (it greater than or equal to 10(18) cm(-1)), we observe similar LO phonon-plasmon (LOP) coupling as in GaAs:Si. Unexpectedly LOP coupling is disorder-cancelled at higher N contents while the pure LO and disorder-activated theoretically forbidden TO modes substitute for it. Disorder-induced red-side LO asymmetries are fairly accounted for by the spatial correlation model with Gaussian distribution and the fitted value of correlation length is a quantitative indication for the disorder rate. (C) 2002 Elsevier Science Ltd. All rights reserved.