SIMS and XPS were used to characterise the chemistry of thin plasma polymerised acrylic acid films (ppAAc), and to determine how this was influenced by plasma power. Quartz microbalance weight measurements were used to monitor the effect of power on the deposition rate and identify the uptake of water vapour by the films upon exposure to the atmosphere. Functional group derivatisation and XPS were used to quantify the proportion of carboxylic acid and ester functionalities. Derivatisation revealed that the level of retention in the deposit could be controlled by the plasma deposition power (P) up to a maximum of 66% at P=2 W. TOF SIMS analysis identified the presence of linear structures with up to five monomer units in the high retention deposit. The role of such structures in functional retention is discussed with reference to mass spectrometry data in the literature.