The peculiar behaviour of the system CsC24 - ethylene, is the ability for ethylene molecules to be oligomerized inside the host material, depending on the temperature. At low temperature, reversible intercalation occurs in two steps depending on the gas pressure, forming respectively a second stage ternary and a first stage ternary, whereas at temperature higher than similar to 250 K, only the second stage ternary is obtained and the reaction remains irreversible. By calorimetric measurement in the range 288 K to 308 K, two peaks were identified, corresponding for the first one to the heat of intercalation, and for the second one, to the oligomerization process. After exposure in air, a part of cesium is oxidised, but the ternary remains intercalated.