Abstract. Weil algebra morphisms induce natural transformations between Weil bundles. In some well known cases, a natural transformation is endowed with a canonical structure of affine bundle. We show that this structure arises only when the Weil algebra morphism is surjective and its kernel has null square. Moreover, in some cases, this structure of affine bundle passes to jet spaces. We give a characterization of this fact in algebraic terms. This algebraic condition also determines an affine structure on the groups of automorphisms of related Weil algebras.Introduction. The theory of Weil bundles and jet spaces is developed in order to understand the geometry of PDE systems. C. Ehresmann formalized contact elements of S. Lie, introducing the spaces of jets of sections; simultaneously A. Weil showed in [8] that the theory of S. Lie could be formalized easily by replacing the spaces of contact elements by the more formal spaces of "points proches", known as Weil bundles. The general theory of jet spaces [6] recovers the classical spaces of contact elements J l m M of S. Lie applying the ideas and methodology of A. Weil.In the theory of Weil bundles, morphisms A → B of Weil algebras induce natural transformations [5] between Weil bundles. There are well known cases in which these natural transformations are affine bundles that often appear in differential geometry [5]. In [4] I. Kolář showed that this is the behaviour of M A l → M A l−1 . In this paper we characterize the natural transformations that are affine bundles. It is done easily by adopting a different point of view on the tangent space of M A than in [6]. Our result is as follows: there is a canonical affine structure for natural transformations M A → M B induced by a surjective morphism A → B whose kernel has null square. This is true for M A l → M A k with 2k + 1 ≥ l > k ≥ 0.In some cases the natural transformations induce maps between jet spaces. This holds in the cases studied in [4]. We characterize this situation, and moreover, we determine when an affine structure on the Weil bundle morphism passes to the jet space morphism. In addition, we prove that in this case there also exists an affine structure in the morphisms be-