Abstract: Time-resolved continuous detection of gas species concentration in breath is provided by optical spectroscopy. Preferably this is performed for both target and reference chemical species, so that the reference measurements can be used to assist in identifying breath cycle phases and to correct the target measurements for contamination by environmental air. Identification of breath cycle phases can be used to gate the data to select the most relevant parts (e.g., the data from when the contribution from alveolar air is maximal).