A detailed understanding of the chemical reactions of organic molecules with semiconductor surfaces will greatly aid schemes for the incorporation of organic functionality into existing technologies. In this paper we report on the reaction of acetaldehyde (CH₃CHO) with silicon (001) as revealed by a combination of temperature-dependent scanning tunneling microscopy (STM) experiments and density functional theory (DFT). We observe that low-coverage exposures at room temperature result almost exclusively in the formation of a single adsorbate species. Conversion of this structure into thermodynamically favored bridge-bonded structures is achieved through temperature anneals between 150–250 °C. We determine the chemical identity of each of the experimentally observed species by comparison with DFT total energy calculations and simulated STM images. Calculations of transition states are used to formulate a full reaction pathway explaining the formation of the observed species. Excellent agreement is found between our experimental measurements and theoretical calculations. The results also present a picture consistent with our previous work on acetone and reveal a general reaction pattern for molecules containing the acetyl COCH₃ functional group, where the initial attachment to the surface is mediated by a carbonyl C=O group. This suggests that modification of the residue R will facilitate in binding other electronically active molecules to the surface in a controlled fashion.