In the offshore exploitation of liquid fuels, problems of line plugging often occur, especially due to gas hydrates crystallisation. At the present time, operators resort to antifreeze additives, which efficiency is defeated either by harder operating conditions or by a more severe environmental legislation. So research recently shifted towards a new class of "low dosage inhibitors". In order to understand the influence of such additives, we designed a high pressure reactor, fitted with a liquid injection device and an in situ turbidimetric sensor. Access to both the particle size distribution (PSD) of the suspension during the first stages of crystallisation, and the total gas consumption, allows us to characterise the kinetics of methane hydrate formation. First, we developed an original experimental procedure, which generates an initial "breeding" of the solution, and thus improves the mastering of nucleation. The induction time then becomes one of the relevant parameters to investigate the performance of inhibitors. Afterwards, we performed a first series of experiments which allowed us to determine the influence of the operating conditions (pressure and stirring) on the evolution of the PSD, in the absence of additives. Then, we pointed out the inhibiting effect of a model kinetic inhibitor, PolyVinylPyrrolidone (PVP). When dissolved in the solution before crystallisation occurs, it increases the induction delay, decreases the gas consumption rate and also slows down the birth of new particles for several hours. On the contrary, when injected in the medium during crystallisation, this polymer no more affects the reaction kinetics. At last, we raise the bases for a modelling, taking into account the elementary crystallisation processes of nucleation, growth and particles agglomeration. A parametric study has been confronted to the experimental data. It enables us to suggest hypotheses regarding the effect of gas hydrates kinetic inhibitors.