The ro-vibrational infrared spectrum of malonaldehyde has been measured for the first time at high resolution. Employing diode laser spectroscopy in a pulsed slit seeded He jet expansion the rotational structure of the nu(6) (OH-bend/symmetric C=C,C=O stretch) fundamental could be completely resolved at a rotational temperature of about 7 K. The rotational structure of the (+) - tunnelling component was analyzed in terms of Watson's semi-rigid rotor Hamiltonian with a band centre at 1594.0898 cm(-1). Additional weaker signals were attributed to the (-) - tunnelling component. The estimated band centre around 1594.06 cm(-1) corresponds to a reduction of the tunnelling splitting upon excitation of nu(6) by only 0.03 cm(-1). (C) 2004 Elsevier B.V. All rights reserved.