The development of predictive models for the long term evolution of nuclear waste glass requires the complete knowledge of the glass dissolution at the laboratory scale. A new approach was developed to determine the initial reaction during the first steps of experience, a new concept was developed, based on the combination of dynamic leaching test and the characterization of the altered materials. With this experimental set-up it is possible to follow in real time the glass alteration process at a fine temporal scale. The results put in evidence a singular behaviour of the lanthanide, shown by a concentration peak of La, Nd and Ce after 2 h and a quick decrease of their concentration measured on line in the solution during the leaching test. This fact is directly linked to the development of an interphase (altered layer which differs from the initial solid by its texture, structure and chemical composition) at the interface of the glass surface and the leaching solution. This work is an attempt to integrate the formation of the alteration products (here the interphase) during leaching into the dissolution mechanisms of a nuclear waste glass. A model is proposed and discussed.