We analyze both the local and nonlocal coupled mode dispersion relations for a degenerate 2D plasma interacting with both surface and bulk optical phonons. The coupled collective modes and their nonlocal shifts, are exhibited as functions of the distance to of the 2D plasma sheet from the bounding surface of the host material. We also explicitly determine the excitation amplitudes (oscillator strengths) of the modes. (C) 1998 Elsevier Science Ltd.