Configurations herein include a novel process, system, etc., to measure a concentration of sulfur trioxide in a gas sample including sulfur trioxide, sulfur dioxide, and water. An example system can include an optical radiation source that generates an optical signal at a plurality of vibration-rotation transitions around at least one frequency selected from the group consisting of 1396.889 cm−1, 1396.908 cm−1, 1396.962 cm−1, 1397.003 cm−1, 1397.037 cm−1, 1397.047 cm−1, and 1397.052 cm−1. The optical signal is transmitted along an optical path through the gas sample. Based on adjusting a pressure of the gas sample, each of the plurality of vibration-rotation absorption transitions associated with the sulfur trioxide can be substantially isolated from interfering absorption by the sulfur dioxide and the water vapor in the gas sample. Based on a spectral analysis, the system generates a value indicative of a concentration of sulfur trioxide in the gas sample.