During high temperature deformation segregation of alloying elements plays a critical role on the performance of superalloys. In these alloys, deformation often occurs under the formation of planar defects such as stacking faults and microtwins. A recently discovered strengthening mechanism, so-called localized phase transformation strengthening, can have either a positive or negative effect, depending on which phases formed along these planar defects during creep. As of now, research on this strengthening mechanism focused on Ni-based superalloys. Therefore, four slightly different single-crystalline CoNi-based superalloys were investigated, to analyze whether localized phase transformation strengthening occurs or not. In addition, the influence of the alloying elements Nb, Ta, Ti and W on the deformation behavior was studied as well. For this purpose, compression experiments at 850°C and 10^-4 s^-1 and compression creep experiments at 750°C and 620 MPa were performed and analyzed via transmission electron microscopy. While all four alloys deformed under stacking fault and microtwin formation, the Ta-containing alloy significantly outperformed the other alloys. Atomic-scale energy dispersive spectroscopy investigations revealed strengthening by local phase transformations along superlattice intrinsic and extrinsic stacking faults as well as microtwins in certain alloys, while the detrimental effect of excessive microtwinning in others. The importance of kinetical aspect on the occurrence of localized phase transformation strengthening will be discussed.