The flow conditions at both the cooling hole exit as well as the cooling hole entrance affect the cooling performance downstream of cylindrical and fan-shaped holes in a very different way. The conseqences of a change in a flow parameter very obviously depend on the respective hole geometry. To gain an as complete as possible view of the specific effects of varying operating conditions and hence an enhanced understanding of the dominating mechanisms, a variation of the hole geometry is imperative. The expansion angle of the diffuser, the inclination angle of the hole, and the length of the cylindrical part at the hole entrance are considered to be the most important geometric parameters of diffuser holes. The effect of a change of exactly these parameters on the film cooling performance will be analyzed. For a better assessment of the characteristic effects associated with contouring of the hole, every diffuser hole is compared to an adequate cylindrical hole. The comparison will be performed by means of discharge coefficients and local and laterally averaged adiabatic film cooling effectiveness and heat transfer coefficients derived from experiments.