Abstract.A methodology and algorithms are proposed for constructing the polynomial chaos expansion (PCE) of multimodal random vectors. An algorithm is developed for generating independent realizations of any multimodal multivariate probability measure that is constructed from a set of independent realizations using the Gaussian kernel-density estimation method. The PCE is then performed with respect to this multimodal probability measure, for which the realizations of the polynomial chaos are computed with an adapted algorithm. Finally, a numerical application is presented for analyzing the convergence properties.Key words. multimodal probability distribution, polynomial chaos, random vector, high dimension AMS subject classifications. 62H10, 62H12, 62H20, 60E10, 60E05, 60G35, 60G601. Introduction. In 1991, R. Ghanem [16] has proposed (1) an efficient construction of the polynomial chaos expansion (PCE) [8] for representing second-order stochastic processes and random fields, and (2) to use it for solving boundary value problems with uncertain parameters by a spectral approach and the stochastic finite elements. Since 1991, numerous works have been published in the area of the PCE and of its use in the spectral approaches for solving linear and nonlinear stochastic boundary value problems, and some associated statistical inverse problems (see for instance [1,9,10,11,13,17,18,27,31,34,42,44]). Several extensions have been proposed concerning generalized chaos expansions, the PCE for an arbitrary probability measure, the PCE with random coefficients [14,28,38,40,49,50], and recently, the construction of a basis adaptation in homogeneous chaos spaces [48]. Although several works have been devoted to the acceleration of stochastic convergence of the PCE (see for instance [19,24,29,48]), the question relative to the speed of convergence (which can be very low) of the PCE for a multimodal probability distribution on R n has been little addressed. Recently, a procedure through mixtures of PCE has been proposed in [33] for the onedimension case. In this paper, we propose a methodology for the PCE of a multimodal R m -valued random variable. This problem belongs to the class of the PCE with respect to an arbitrary probability measure. The framework of the developments presented in the paper is motivated by the difficulty encountered for the PCE of a random vector for which its probability density function is multimodal, and for which it is known that the speed of convergence of the PCE can be low. Nevertheless, the method proposed is very general and goes beyond multimodality. In the context of the statistical inverse problem related to the identification of a PCE of a random vector, one does not know if the unknown probability density function is unimodal or is multimodal. So the method proposed allows for accelerating the speed of convergence in all the cases. We propose an algorithm for generating independent realizations of the multimodal probability measure on R n , which is constructed from a set of realizations us...