A comparison of optimized trilateral cycle (TLC) - systems with water as working fluid and optimized organic Rankine cycle (ORC) systems with pure organic working fluids is presented. The study includes the heat transfer to and from the cycles. The TLC - systems were optimized by the selection of the maximum water temperature, the ORC - systems by the selection of the working fluid and the process parameters. The optimization criterion is the exergy efficiency for power production being the ratio of the net power output to the incoming exergy flow of the heat carrier. Results will be presented for five different cases specified by the inlet temperature of the heat carrier and the inlet temperature of the cooling agent. The inlet temperature pairs are (350 degrees C, 62 degrees C), (280 degrees C, 62 degrees C), (280 degrees C, 15 degrees C), (220 degrees C, 15 degrees C) and (150 degrees C, 15 degrees C). It is found that the exergy efficiency for power production is larger by 14%-29% for the TLC than for the ORC. On the other hand, the outgoing volume flows from the expander are larger for the TLC than for the ORC by a factor ranging from 2.8 for the first case to 70 for the last case. (C) 2011 Elsevier Ltd. All rights reserved.