Dielectric spectra were obtained at ambient and elevated pressures for three 'type-A' glass formers, which exhibit excess intensity on the high frequency side of the structural relaxation peak. The response to pressure of the peak maximum and the excess wing suggests categorization of such glass formers into two groups: associated liquids, in which the alpha-relaxation and the excess wing have a different pressure dependence, and van der Waals liquids, which at fixed value of the alpha-relaxation time, conform to temperature-pressure superpositioning. This distinction is believed to arise from the change in the number of intermolecular bonds (non-dispersive interactions) with volume. (C) 2002 Elsevier Science B.V. All rights reserved.