Freeze/thaw damage in concrete is by general practice concluded to be a problem that can be avoided by using air-entraining agents to develop an air bubble structure in the hardened concrete together with the use of a relatively low water to cement ratio in mix. This fact is true for inner damages, however, the so-called salt-frost damage, occurring mainly at the surfaces of concrete constructions, can not totally be avoided by the above mentioned method. The performance and the mechanisms occurring in concrete, with a substantial amount of water in its micro-structure, at very low temperature are, however, in most part unknown. In this work samples of concrete at different water to cement ratios and air bubble contents subjected to freeze/thaw cycles with the lowest temperature at about -80 oC are investigated. By adopting a novel technique a scanning calorimeter is used to obtain data from which the ice contents at different freeze temperatures can be calculated. The length change caused by temperature and ice content changes during test is measured by a separate experiment using the same types of freeze-thaw cycles as in the calorimetric tests. In this way it was possible to compare the amount of formed ice at different temperatures and the corresponding measured length changes. The development of cracks in the material structure was indicated by an ultra-sonic technique by measuring on the samples before and after the freeze thaw tests. Further the air bubble structure was investigated using a microscopic technique in which air bubble size distributions and the so-called spacing factor, indicating the mean distance between air bubbles, were measured. By analyzing the experimental result it is concluded that damages occur in the temperature range of about -10 oC to 55 oC, when the air content is lower than about 4% of the total volume. For a totally water saturated (using vacuum) concrete damages always occur independently of the use of entrained air or low water to cement ratios. It is, further, concluded that the length changes of theses samples corresponds to the calculated ice contents at different temperatures in a linear fashion. Even though a quite extensive experimental investigation are performed in this work, it is concluded to be difficult to verify or reject the three most common theories for the mechanisms of inner frost damage, i.e. the closed container-, hydraulic pressure- and ice lens growth theory. The new suggested method to calculate the ice contents at different temperatures, using raw data from the adopted scanning calorimetric technique, are however ideal suited for this purpose once more directed experiments are performed.