This communication tried to describe and explain the selling and purchasing practices between energy suppliers (gas and electricity) and industrial customers in France. It reports on the diffusion of different transactions practices, in terms of organisation of call for tenders, consultations, consumption prevision, price hedging (fixed prices, indexed prices, with different price references), and on alternatives like purchasing consortium and regulated tariffs. The communication refers to two theoretical frameworks: the sociology of market questions the conditions under which energy suppliers and industrial customers are calculating and trading practically; the neo-institutional organizational theory questions the emergence and the competition between alternatives transactions practices. Our research reveals the importance of “calculation capabilities” as a condition of institutionalisation of a new market, and show that this condition isn't valid already on the French energy markets.