The low temperature luminescence spectrum of silicon doped with beryllium has been found to contain a strong set of no-phonon lines centred around 1-077 eV with phonon assisted replicas at lower energies. The energies of these transitions are as would be expected for an isoelectronic bound exciton (IBE). The thermalization of the no-phonon lines follows the pattern of an exciton bound to an axially symmetric centre.It has already been proposed that when Be is doped into Si the majority of the Be forms substitutional-interstitial pairs, which would be axial isoelectronic centres. The strength and lifetimes of the absorption lines of the centre are consistent with the IBE assignment. Zeeman studies of the luminescence spectra show that the centre has a <100> axis. The temperature variations in the phonon sidebands are not obviously distinctive of either a donor or an acceptor-like IBE. However while the series of excited states of the exciton as observed in absorption cannot be fitted to a donor series it is found to match exactly that of the boron acceptor in silicon. In this study the absorption spectrum of the centre is examined under uniaxial stress to try to determine conclusively the nature, whether donor or acceptor-like, of the centre. The absorption lines of interest coincide with a region of strong water vapour absorption and under stress many of the lines are extremely weak- To obtain usable data the spectra are normalised with respect to a previously recorded standard containing only the water vapour absorption. This process is performed by a microcomputer and not only greatly enhances the features of interest but also removes the need to calibrate the system as any variations with wavelength in the output of the light source, the response of the spectrometer and the sensitivity of the detector are compensated for automatically. Several aspects of the results obtained are examined and all are found to be consistent with the acceptor-like classif ication.