  Using a data sample of 2.59 x 10^7 psi(2S) decays obtained with the CLEO-c
detector, we perform amplitude analyses of the complementary decay chains
chi_c1 -> eta pi+ pi- and chi_c1 -> eta' pi+ pi-. We find evidence for a P-wave
eta' pi scattering amplitude, which, if interpreted as a resonance, would have
exotic J^PC = 1^-+ and parameters consistent with the pi_1(1600) state reported
in other production mechanisms. We also make the first observation of the decay
a_0(980) -> eta' pi and measure the ratio of branching fractions B(a_0(980) ->
eta' pi)/B(a_0(980) -> eta pi) = 0.064 +- 0.014 +- 0.014. The pi pi spectrum
produced with a recoiling eta is compared to that with eta' recoil.
