Let \(f(t_1, \ldots , t_r, X)\in {\mathbb {Z}}[t_1, \ldots , t_r,X]\) be irreducible and let \(a_1, \ldots , a_r\in {\mathbb {Z}}{\setminus } \{0,\pm 1\}\) . Under a necessary ramification assumption on f, and conditionally on the Generalized Riemann Hypothesis, we show that for almost all integers \(n_1, \ldots , n_r\) , the polynomial \(f(a_1^{n_1}, \ldots , a_r^{n_r}, X)\) is irreducible in \({\mathbb {Q}}[X]\) .