Relative intensities in the vibrational structure of the Ã(1)A(u)(C(2h))-&Xtilde;(1)Sigma(+)(g)(D(infinityh)) electronic transition of acetylene are calculated. The calculation takes account of the large change of geometry and the change in the normal coordinates (the Dushinskií effect) between the two states. Because conventional vibrational wavefunctions for a nonlinear state do not behave correctly at linear geometries the vibrational integrals are only evaluated approximately. The transition ((1)Sigma(u)(-)-(1)Sigma(+)(g)) is forbidden at linear geometries, and so calculations are performed without and with a factor proportional to the angle of bend from linearity. Good agreement with experiment is obtained for the first few quanta of the absorption spectrum, lending support to the Ã-state harmonic potential of Tobiason et al., J. Chem. Phys. 99, 5762 (1993). Qualitative agreement is obtained for the emission spectrum (Jacobson and Field, J. Phys. Chem. 104, 3073 (2000)) when allowance is made for anharmonicity in the &Xtilde; state, but more quantitative agreement will require improvements in the empirical potential of the &Xtilde;-state, or an ab initio potential to higher energies than available at present. Copyright 2001 Academic Press.