The ν5 and ν6 fundamental bands of formaldehyde-d1 vapour have been recorded by conventional infrared absorption spectroscopy with a spectral resolution of ~0.05 cm−1. The excited vibrational levels are strongly coupled by a- and b-type Coriolis interactions and must be analysed simultaneously. Such an analysis has been performed using the present results and the excited state microwave data, and the resulting band origins, rotational, centrifugal distortion, and Coriolis coupling parameters are given. The HDCO spectrum in the 9–11 μm region is of special interest since deuterium isotope separation has been demonstrated using 10.6 μm CO2 lasers. Because of this interest, the complete observed spectrum, a listing of observed lines, and some other lengthy tables are collected in an appendix to this work.