Film cooling, as a key technology to ensure turbine survival in new generation gas turbines, has been studied profusely in subsonic flows. But in transonic flow, the determination of adiabatic cooling effectiveness faces a dilemma in the state of the art. Specifically, derivation of reference temperature, or local recovery temperature, in adiabatic effectiveness is disputable. Some researchers designate it to be the adiabatic wall temperature for the uncooled model (linear regression method (LRM)), but others calculate it from an iterative procedure based on a pair of cooling tests (dual linear regression technique (DLRT)). As the first of the kind effort to explore this dilemma, this article carried out transient thermal measurements by infrared thermography, for transonic flow over an idealized blade tip model. Heat transfer experiments were conducted for the uncooled and cooled cases, at two mainstream temperatures of 340 K and 325 K and two coolant temperatures of 276 K and 287 K. Data from these six experimental groups were processed by LRM and DLRT, respectively, to obtain heat transfer coefficient and adiabatic effectiveness, whose sensitivity to mainstream and coolant temperatures is tested and compared. It is found that the heat transfer coefficient is basically insensitive to temperature boundary conditions and data reduction methods, as expected. However, for adiabatic effectiveness, LRM results are sensitive to the 11 K decrease of coolant temperature in areas confined to the upstream of cooling injection, and much less so to the 15 K rise in mainstream temperature. DLRT result, derived from the test pair with two coolant temperatures, reduces globally and conspicuously with 15 K increase in mainstream temperature. Furthermore, adiabatic effectiveness obtained by LRM is qualitatively different from that by DLRT, which is mainly attributed to the large discrepancy in reference temperature between the two methods.