The beginning of the 21th century was marked by an increase in the usage of natural gas for electricity and thermal energy. The upward trend was accompanied by raised awareness when it comes to Earth’s ecological capacities and the need to protect the environment as well as taking into account the concept of sustainable development. All this resulted in a sudden increase in the transportation of natural gas through pipelines. The gas transmission system is the central part of the gas market, which is also the case in the Croatian gas market. It connects all the other participants from the gas sector and it enables the flow of gas from the production facilities to its consumers, who are the actual force setting the market in motion. There is a large number of processes which are happening along the way and relations between members of the market are being negotiated. These contracts are in accordance with the CAM and the BAL regulations and other EU statutes. The transmission system capacity booking and the nomination of the transmission system use are the prerequisites in order for all the market subjects to be able to operate. In other words, this is related to the transmission and distribution of gas. By contracting capacities which are offered by the operator of the transmission system, the right to a certain technical capacity of the transport system is acquired, and the nomination verifies and enforces this right. When the transmission is materialized, and in regard to the liberalization process of the Croatian market and the large number of active members, the delivered gas energy has to be divided among the users of the system, which is not an easy task since daily measuring are lacking. Plinacro, as the operator of the Croatian transmission system, has started the test phase of the newly-developed allocation mechanism so as to improve their service for all the users, as well as to optimize the system which they run, monitor and balance. By doing this, they are laying the groundwork for the interaction between other members on the gas market.