High temperature heat pumps (HTHP) with heat sink temperatures of 100 to 160 °C will increasingly become commercialized in the coming years, especially for industrial drying, sterilization and evaporation processes. In particular, the HFO R1336mzz(Z) and the HCFOs R1233zd(E) & R1224yd(Z) are low GWP replacement refrigerants for R245fa and R365mfc. This study examines the experimental performance of R1336mzz(Z) and R1233zd(E) in a laboratory HTHP with 5 to 10 kW heating capacity. The developed heat pump is single-stage, operates with a variable speed piston compressor, and contains a continuously adjustable internal heat exchanger (IHX) for superheating control. The performance data were measured at 30, 50, and 70 K temperature lift (40 to 80°C heat source, 80 to 150°C heat sink). At W60/W110, the experimental results with R1233zd(E) showed a COP of 2.8 (basic cycle) and 3.1 with IHX integration (+15%). For R1336mzz(Z) a COP of 2.4 and 3.0 was reached (+24%). By increasing the heat sink temperature glide from 5 to 30 K, a further 15% COP increase was achieved.