The reliability of a gas turbine power plant is typically much higher than that of a hydroelectric plant, especially that at Pak Mun which is characteristically run-ofthe-river with restrictive operating rules as agreed conditions for environmental concerns. Assuming a 98% norm of operating reliability,17/ an equivalent dependable capacity of 75 MW for an internal combustion gas turbine plant might therefore require a design capacity rated at only 76.5 MW. This lower rating of required capacity, as against 150 MW used by EGAT and the World Bank in their respective appraisals, would have meant lower capital investment and operating costs for the alternative plant used as proxy for determining Pak Mun’s economic benefits on the basis of avoided costs. Other assumptions being the same, using a 76.5 MW installed capacity for the alternative gas turbine plant at 1987 prices instead of 136 MW reduces the project’s EIRR as a power project from 19.8% to 15.7%. Similarly, using a 76.5 MW capacity instead of 150 MW for the alternative gas turbine plant at 1991 prices reduces Pak Mun’s EIRR from 17.9% to 13.2%.