The long-term nonlinear evolution of monochromatic large-amplitude Alfv¿n waves in oblique propagation is studied via one-dimensional magnetohydrodynamic (MHD) numerical simulations. Arc-polarized waves, which are exact solutions for non-parallel propagation and are observed in solar wind data, are shown to be subject to parametric decay instabilities just like the well studied case of circularly polarized waves, not only when arc polarization is used as initial condition but also when this state is reached after nonlinear evolution of linearly polarized waves. ¿ 2001 American Geophysical Union