THAI-"Toe-to Heel Air Injection" is an integrated horizontal process for the recovery and in situ upgrading of heavy crude oil. In addition to thermal upgrading, further upgrading can be achieved by emplacing a catalyst layer around the horizontal producer well, in effect creating a downhole reactor: A series of six catalytic tests was performed in two 3D combustion cells to investigate the degree of upgrading achievable with 10.5degrees API gravity Wolf Lake oil. The main factors investigated were the type of HDS catalyst (NiMo, CoMo), extrudate or crushed catalyst, and catalyst loading. The results of these tests show that the basic process is very stable over a 10-hour period, maintaining an average peak combustion temperature of 500 to 600 degreesC. Thermal upgrading alone (THAI) achieved nearly 10 points increase in the API gravity of the produced oil. This was increased by a further 4 to 7 API using the catalytic process. This indicates that it may be possible to convert a heavy crude oil to almost a light oil product in a one-step in situ reservoir process. The oil recoveries were consistently very high, at around 85% OOIP, with the produced oil having a viscosity as low as 10 mPa.s at 20 degreesC, (i.e., mPa.s). The process demonstrated robust stability when changes in the air injection rate were made, and there was also an oil productivity gain when operating at a low water air ratio, WAR < 1.0.