The plasma polymerization of acrylic acid has been probed by means of mass spectrometry (neutrals and positively charged species) and ion energy spectroscopy. The interpretation of mass spectra is facilitated by the comparison of mass spectra obtained from plasmas of a C-13-labeled acrylic acid (with the label at the carboxyl C) and plasmas of an unlabeled acrylic acid. Mass spectrometry of the neutrals shows that under the action of plasma, even at low plasma power inputs, P, there is substantial fragmentation of the acrylic acid. Fragmentation is homolytic at the carboxyl functional group, and this is attributed to energy transfer from electrons. In the positive ion mass spectrometry, oligomers of the series (nM + H)(+) as high as n = 4 were detected. Low ion energies were measured at low P and are consistent with minimal fragmentation on arrival at a self-biased surface. On the basis of these measurements, we speculate that at low P, gas-phase cationic oligomers may be responsible for up to 50% of the deposit＇s total mass. Grafting of "intact" acrylic acid accounts for the remaining mass, and the two processes (gas-phase oligomerization and grafting) give rise to a highly functionalized. plasma polymer, containing predominately carboxyls. Irrespective of the exact weighting given to these alternative mechanisms, it is the abundance of intact acrylic acid which determines the extent of functional group retention. It follows that as P increases and the amount of intact acrylic acid declines steeply, high levels of functional group retention cannot be achieved.