We study the problem of 2-soliton collision for the generalized Korteweg-de Vries equations, completing some recent works of Y. Martel and F. Merle [24,25]. We classify the nonlinearities for which collisions are elastic or inelastic. Our main result states that in the case of small solitons, with one soliton smaller than the other one, the unique nonlinearities allowing a perfectly elastic collision are precisely the integrable cases, namely the quadratic (KdV), cubic (mKdV) and Gardner nonlinearities.Here u = u(t, x) is a real-valued function, and f : R → R a nonlinear function, often refered as the nonlinearity of (1). This equation represents a mathematical generalization of the Korteweg-de Vries equation (KdV), namely the case f (s) = s 2 ,(2) other physically important cases are the cubic one f (s) = s 3 , and the quadratic-cubic nonlinearity, namely f (s) = s 2 −µs 3 , µ ∈ R. In the former case, the equation (1) is often refered as the (focusing) modified KdV equation (mKdV), and in the latter, it is known as the Gardner equation. Concerning the KdV equation, it arises in Physics as a model of propagation of dispersive long waves, as was pointed out by J. S. Russel in 1834 [27]. The exact formulation of the KdV equation comes from Korteweg and de Vries (1895) [15]. This equation was re-discovered in a numerical work by N. Zabusky and M. Kruskal in 1965 [34].After this work, a great amount of literature has emerged, physical, numerical and mathematical, for the study of this equation, see for example [5,14,30,18,9,28,27]. Although under different points of view, among the main topics treated are the following: existence of explicit solutions and their stability, local and global well posedness, long time behavior properties and, of course, related generalized models, hierarchies and their properties.This continuous, focused research on the KdV equation can be in part explained by some striking algebraic properties. One of the first properties is the existence of localized, rapidly decaying, stable and smooth solutions called solitons. Given three real numbers t 0 , x 0 and c > 0, solitons are solutions of (2) of the form u(t, x) := Q c (x − x 0 − c(t − t 0 )), Q c (s) := cQ(c 1/2 s),and where Q satisfies the second order nonlinear differential equation Q ′′ − Q + Q 2 = 0, Q(x) = 3 2 cosh 2 ( 1 2 x).