We develop differential calculus of C r,s -mappings on products of locally convex spaces and prove exponential laws for such mappings. As an application, we consider differential equations in Banach spaces depending on a parameter in a locally convex space. Under suitable assumptions, the associated flows are mappings of class C r,s .MSC 2000 Subject Classification: Primary 26E15; Secondary 26E20, 46E25, 34A12, 22E65, 46T20 (A. Schmeding) spaces, U ⊆ E 1 and V ⊆ E 2 be open subsets and r, s ∈ N 0 ∪ {∞}. We say that a map f :exist for all for all i, j ∈ N 0 with i ≤ r and j ≤ s, and are continuous functions in (x, y, w 1 , . . . , w i , v 1 , . . . , v j ) ∈ U ×V ×E i 1 ×E j 2 (see Definition 3.1 for details). To enable choices like U = [0, 1], and also with a view towards manifolds with boundary, more generally we consider C r,s -maps if U and V are locally convex (in the sense that each point has a convex neighbourhood) and have dense interior (see Definition 3.2). These properties are satisfied by all open sets. Variants and special cases of C r,s -mappings are encountered in many parts of analysis. For example, [2] considers analogues of C 0,r -maps on Banach spaces based on continuous Fréchet differentiability; see [12, 1.4] for C 0,r -maps; [14] for C r,s -maps on finite-dimensional domains; and [11, p. 135] for certain Lip r,s -maps in the convenient setting of analysis. Cf. also [32], [20] for ultrametric analogues in finite dimensions. Furthermore, a key result concerning C r,s -maps was conjectured in [15, p.10]. However the authors' interest concerning the subject was motivated by recent questions in infinite dimensional Lie theory. At the end of this section, we present an overview, showing where refined tools from C r,s -calculus are useful. The first aim of this paper is to develop necessary tools like a version of the Theorem of Schwarz and various versions of the Chain Rule. After that we turn to an advanced tool, the exponential law for spaces of mappings on products (Theorem 3.28). We endow spaces of C r -maps with the usual compact-open C r -topology (as recalled in Definition 2.5) and spaces of C r,s -maps with the analogous compact-open C r,stopology (see Definitions 3.21 and 4.3). Recall that a topological space X is called a k-space if it is Hausdorff and its topology is the final topology with respect to the inclusion maps K → X of compact subsets of X (cf.[26] and the work [39], which popularized the use of k-spaces in algebraic topology). For example, all locally compact spaces and all metrizable topological spaces are k-spaces. The main results of Section 3 (Theorems 3.25 and 3.28) subsume:Theorem A. Let E 1 , E 2 and F be locally convex spaces, U ⊆ E 1 and V ⊆ E 2 be locally convex subsets with dense interior, and r, s ∈ N 0 ∪ {∞}. Then γ ∨ : U → C s (V, F ), x → γ(x, •) is C r for each γ ∈ C r,s (U × V, F ), and the map Φ : C r,s (U × V, F ) → C r (U, C s (V, F )), γ → γ ∨ (1)is linear and a topological embedding. If U × V × E 1 × E 2 is a k-space or V is locally compact, then Φ is...