We have measured Raman, optical absorption, and fluorescence spectra of 9, 10-diiodoanthracene (DIA) crystals as a function of hydrostatic pressure. Unlike analogous anthracene derivatives with similar quasi-one-dimensional stacking of molecules, the ambient pressure crystal structure remains stable up to at least 20 GEPa. Lattice dynamics calculations, based on intermolecular potentials, reproduce well the structural parameters at zero pressure and the pressure dependence of the intermolecular Raman-active phonons. An explanation is given for the unusual stability of the one-dimensional molecular packing in DIA. (C) 1997 American Institute of Physics.