A method for determining a sulfur trioxide content in a gas. A sample of the gas is taken, and a gas pressure of the sample is reduced. A wave number-resolved transmission measurement is carried out on the sample using a wave number-tunable monochrome light source, and a sulfur trioxide content is derived from the measurement. The measurement is carried out in the sulfur trioxide absorption band between 1360 and 1410 cm−1, in particular in a window around the sulfur trioxide absorption at 1365.49 cm−1. A method for operating a power plant, a measuring system for determining a sulfur trioxide content in a gas, and a power plant are adapted to determine a sulfur trioxide content in a gas.