This paper describes some of the major issues related to catalytically ignited premixed combustion and the effect of pressure and velocity on the mass transfer of the catalytic reaction. The first issue is concerned with partial burning of the fuel air mixture on the catalyst surface prompting gas phase combustion to take place downstream of the catalyst layer. The latter part of this paper deals with the potential for reduced combustion efficiency from a mass transfer perspective, when going from atmospheric testing conditions to actual high pressure gas turbine operating conditions.