Temperature measurements in the test section of a hypersonic gun tunnel using a modified line-reversal two-beam optical pyrometer have been performed. Measurements were made in the shock layer at the front of an ‘end-on’ flat-faced cylinder using argon or air as the driven gas. The measured temperatures were greater than the calculated isentropic temperatures but much less than the originally predicted shock-heated values. The experimental values agreed reasonably well with the theoretical temperatures for shock-heated air corrected for boundary-layer and heat-loss effects in the gun barrel and also with previously reported experimental values found using heat-transfer methods. Preliminary studies of the vibrational relaxation of nitrogen and air in the test section have been carried out using a flat-sided ‘relaxation’ tube (a Pitot tube with glass windows). They demonstrated the feasibility of this approach.