A turning diffuser is often introduced in the flow line to recover the energy losses by converting the kinetic energy to pressure energy. There are two types of turning diffusers, i.e. a 2-D and 3-D diffuser that are commonly defined by their expansion direction. This study aims to investigate the performance of a 2-D and a 3-D turning diffuser with 90° angle of turn and an area ratio, AR=2.16 by means of varying operating conditions. The geometry configurations applied for a 2-D turning diffuser are outlet-inlet configurations, W2/W12-D=2.160, X2/X12-D =1.000 and an inner wall length to an inlet throat width ratio, Lin/W12-D=4.370, whereas for a 3-D turning diffuser, they are W2/W13-D=1.440, X2/X13-D =1.500 and Lin/W13-D=3.970. The operating conditions represented by inflow Reynolds numbers, Rein are varied from 5.786E+04 to 1.775E+05. Particle image velocimetry (PIV) is used to examine the flow quality, and a digital manometer provides the average static pressure at the inlet and outlet of the turning diffuser. A compromise between the maximum permissible pressure recovery and flow uniformity is determined based upon the need. Whenever the flow uniformity being the need it is promising to apply a 3-D turning diffuser for Rein=1.027E+05 - 1.775E+05 and a 2-D turning diffuser for Rein=5.786E+04-6.382E+04. On the other hand, it is viable to opt for a 3-D turning diffuser for Rein=5.786E+04-6.382E+04 and a 2-D turning diffuser for Rein=1.027E+05-1.775E+05 in the case of the outlet pressure recovery being the need. The secondary flow separation takes place prior at 1/2Lin/W1 for a 2-D turning diffuser, whereas approximately at 3/4Lin/W1 for a 3-D turning diffuser. © (2014) Trans Tech Publications, Switzerland.