Optimization studies along with optimum parameter correlations, using constant area mixing model are presented in this article for ejector-expansion transcritical CO2 heat pump cycle with both conventional and modified layouts. Both the energetic and exergetic comparisons between valve, turbine and ejector-expansions-based transcritical CO2 heat pump cycles are also studied for simultaneous cooling and heating applications. Performances for conventional layouts are presented by maximum COP, optimum discharge pressure and corresponding entrainment ratio and pressure lift ratio of ejector, whereas for modified layout by maximum COP, optimum discharge pressure and corresponding pressure lift ratio. The optimization for modified layout can be realized for certain entrainment ratio, evaporator and gas cooler exit temperature combinations. Considering the trade-off between the system energetic and exergetic performances, and cost associated with expansion devices, the ejector may be the promising alternative expansion device for transcritical CO2 heat pump cycle. (C) 2008 Elsevier Ltd. All rights reserved.