Abstract : We have studied the role of the disorder on the behaviour of the magnetic field induced spin density waves (FISDW). The experiments were performed on a (TMTSF)2ClO4 single crystal. The low temperature properties of this material are strongly affected by the cooling rate through the anion-ordering transition at 24 K. The effect of this kinetic phenomenon is to freeze partial disorder in the sample at low temperature. Thanks to the use of this supercooling effect, the rate of disorder can be controled in the sample. Our investigations are conducted by means of simultaneous measurements of the specific heat and the magnetocaloric effect, as functions of the magnetic field, and by mean of measurements of the specific heat at constant field as function of the temperature. The magnetic field and the temperature ranges of the measurements are respectively, from 3 to 7 teslas, and from 0.4 to 2 kelvins. We have found a new multicritical behaviour at one point of the second order phase transition line wich separates the metallic phase from the FISDW sub-phases. The criticality of this point changes from "tetracritical" to bicritical as the anion disorder increases. The existence of the "tetracritical" point can be interpreted as the result of the superposition of adjacent FISDW. We have also reported a pair-breaking effect induced by the non magnetic disorder. This pair-breaking differs strongly from the universal behaviour. Moreover, the departure depends on the magnetic field, and is not monotonic as function of the field. These measurements show also that the anion ordering does not affect the re-entrance of the metallic state into the FISDW. From our results, we can deduce that, in the magnetic field range we have investigated, the anion gap opened at the ordering transition is not directly responsible for the re-entrance phenomenon.