An experimental investigation of the thermal blooming of a focused 3 mu s single pulse CO₂ laser beam is described. An absorption cell, containing a propane-nitrogen gas mixture at atmospheric pressure was placed with its exit aperture in the focal plane of the laser beam. Measurements were made over a range of absorption coefficients from 2.4*10^-4 cm^-1 to 2.3*10^-3 cm^-1 and laser energies from 1.5 J to 2.4 J using an eight element linear array of detectors positioned across the beam diameter. Thermal blooming results are shown for times up to 2 mu s after the start of the laser pulse. This spans the focal plane hydrodynamic time of approximately 1.7 mu s. These results are compared with theoretical predictions obtained using a full wave, full hydrodynamic computer code, in which care was taken to accurately model the laser beam. The theoretical calculations consistently overpredicted the changes in the laser profile due to blooming. It is concluded that the most likely explanation of this discrepancy is that more accurate modelling of the laser beam spatial profile is required.