Abstract. We say that the series of general term u n = 0 is fast converging if log |u n | ≤ c2 n for some c < 0. We prove irrationality results and compute irrationality measures for some fast converging series of rational numbers, by using Mahler's transcendence method in the form introduced by Loxton and Van der Poorten. With very weak assumptions on sequence u n , this method allows to obtain only irrationality results.