The structure and energy properties of the 1:1 complex formed between HCl and carbon suboxide C3O2 (3) have been investigated using FTIR matrix isolation spectroscopy and ab initio calculations at the MP2/6-31G** level. Investigations have been extended to other oxocumulenes such as H2C=C=O (1), and (CH2)(2)-CCO (2) complexed by HCl. Trapped in argon matrixes, these complexes are characterized by a large shift of the v(HCl) stretching mode to lower frequencies (191, 135, and 150.5 cm(-1) for 1-3, respectively). The similarity of IR spectra of the complexes generated in the gas or in the solid phase is indicative of a similar inclusion site. Concordance between experimental and calculated v(HCl) shifts is obtained with T-shaped complexes. These complexes describe the interaction between hydrogen and the C-beta carbon atom of the ketene moiety. The geometrical modifications of the ketene subunit suggest that the complex is the product-like transition state of the corresponding electrophilic addition to the carbon atom.