Enthalpy quantities such as the degree of moisture of gases, the wetness of steam or the enthalpy of dry steam are measured by passing a specific amount or dosage of a gas or steam through a system such as a cooler or dryer wherein its thermal condition is changed while measuring the inlet and outlet temperature, and measuring heat radiated into the condition changing system whereupon the desired enthalpy quantity is obtained from a heat balance equation set up from the measured and known data.