A motor vehicle engine compartment comprising: - a support structure comprising a wall having: a) at least one opening, and b) at least one retaining seat; - an expansion tank for a cooling system, the expansion tank comprising: a) a container adapted to receive a cooling fluid; b) connecting means for fixing said container to said support structure; said connecting means comprising at least one bracket, which is fixed with respect to said container, laterally protrudes with respect to said container and comprises: bayonet coupling means defined by at least one connecting appendix, engaging said opening and configured so as to couple said bracket to said support structure by way of a first mounting movement, which is directed vertically downwards and allows said bracket to be rested onto said wall, and a subsequent second mounting movement, which is directed along a horizontal direction and allows said bracket to be vertically locked; at least one locking tooth elastically deformable and adapted to engage snap-wise said retaining seat for locking said bracket along the said horizontal direction characterized in that: - the connecting means comprise a positioning pin which protrudes vertically downwards; and - said support structure comprises a horizontal protrusion having a vertical hole, which is engaged by said positioning pin and defines an elongated slot along a horizontal axis parallel to the horizontal direction of the second mounting movement. - a support structure comprising a wall having: a) at least one opening, and b) at least one retaining seat; a) at least one opening, and b) at least one retaining seat; - an expansion tank for a cooling system, the expansion tank comprising: a) a container adapted to receive a cooling fluid; b) connecting means for fixing said container to said support structure; said connecting means comprising at least one bracket, which is fixed with respect to said container, laterally protrudes with respect to said container and comprises: bayonet coupling means defined by at least one connecting appendix, engaging said opening and configured so as to couple said bracket to said support structure by way of a first mounting movement, which is directed vertically downwards and allows said bracket to be rested onto said wall, and a subsequent second mounting movement, which is directed along a horizontal direction and allows said bracket to be vertically locked; at least one locking tooth elastically deformable and adapted to engage snap-wise said retaining seat for locking said bracket along the said horizontal direction characterized in that: a) a container adapted to receive a cooling fluid; b) connecting means for fixing said container to said support structure; said connecting means comprising at least one bracket, which is fixed with respect to said container, laterally protrudes with respect to said container and comprises: bayonet coupling means defined by at least one connecting appendix, engaging said opening and configured so as to couple said bracket to said support structure by way of a first mounting movement, which is directed vertically downwards and allows said bracket to be rested onto said wall, and a subsequent second mounting movement, which is directed along a horizontal direction and allows said bracket to be vertically locked; at least one locking tooth elastically deformable and adapted to engage snap-wise said retaining seat for locking said bracket along the said horizontal direction characterized in that: bayonet coupling means defined by at least one connecting appendix, engaging said opening and configured so as to couple said bracket to said support structure by way of a first mounting movement, which is directed vertically downwards and allows said bracket to be rested onto said wall, and a subsequent second mounting movement, which is directed along a horizontal direction and allows said bracket to be vertically locked; at least one locking tooth elastically deformable and adapted to engage snap-wise said retaining seat for locking said bracket along the said horizontal direction characterized in that: - the connecting means comprise a positioning pin which protrudes vertically downwards; and - said support structure comprises a horizontal protrusion having a vertical hole, which is engaged by said positioning pin and defines an elongated slot along a horizontal axis parallel to the horizontal direction of the second mounting movement.