Abstract: There is provided an improved system for expired gas analysis indirect analysis indirect calorimetry comprising a unique compliant mixing chamber that is directly connected to a mouthpiece that, itself, is unique in providing enhanced saliva and expired moisture trapping capability. The volume of the inspired and expired gas as well as at least the oxygen and carbon dioxide concentrations in the expired gas in the mixing chamber are determined. The metabolic rate of the subject is calculated therefrom taking into account the anatomical dead space in the body of the subject. The mixing chamber is compliant, sized to accommodate a single expiration and partially open to the environment. Preferably, the system also includes means to determine the oxygen and carbon dioxide content of the inspired gas as well.