Eigenvalues and wave functions for acetaldehyde in its electronic ground state and the (n,3s) Rydberg state are calculated using the rho-axis method in combination with the high barrier approximation. The rotational structure of the first members of the torsional sequence in the two-photon spectrum is analyzed in terms of a zeroth-order rotation-torsional Hamiltonian. Combining experimental information on band positions and rotational contours, we determine the range of internal rotation constants and barrier heights, which is compatible with the experimental data. The analysis yields the following parameters : F= 8.45+/-0.15 cm(-1), V-3=809+/-10cm(-1), rho =0.33+/-0.03, Theta(RAM)=4.5 degrees+/-0.8 degrees, and A(PA)=1.990 cm(-1), B-PA=0.330 cm(-1), C-PA=0.298 cm(-1)