Let X be a projective smooth irreducible polarized variety over the field of complex numbers. Typical examples of wide extensions are vector bundles E that have a subsheaf F whose slope is much bigger than the slope of E/F, and such that F and E/F are stable. We study the deformations of such bundles. The case of unstable rank 2 bundles has been considered by S. A. Strømme on ސ 2 , and by C. Bȃnicȃ on ސ 3 . We build moduli spaces of wide extensions, and if the dimension of X is greater than 2, it may even happen that we obtain fine moduli spaces.