Plasma polymerised acrylic acid (ppAAc) coatings were deposited on aluminium substrates from acrylic acid plasmas at various plasma powers (P). The chemistry of the ppAAc deposit was characterised using X-ray photoelectron spectroscopy (XPS) and trifluoroethanol derivatisation of the carboxylic acid functionalities. The absorption of atmospheric moisture by ppAAc was quantified using a vibrating quartz crystal microbalance. The solubility in ethanol, water and hexane of ppAAc deposited on aluminium was investigated using substrate signal attenuation in XPS. The high carboxylic acid concentrations retained in the low P deposits correlates with a hydrophilic ppAAc coating, i.e. they are readily soluble in water and absorb moisture from the atmosphere. The lower carboxylic acid content and greater crosslinking of coatings produced at higher P result in less hydrophobic deposits which are resistant to rinsing with water. Addition of 1,7-octadiene vapour to a P = 2 W acrylic acid plasma produces insoluble plasma polymerised acrylic acid-co-1,7-octadiene (ppAAc-co-Oct) deposits. A carboxylic acid containing fraction of these insoluble coatings is removed by hexane rinsing which we propose is physisorbed oligomer. We propose that slow evaporation of these oligomers is responsible for the decrease in the acidity of the coatings observed over time under atmospheric storage. The contact angle of water insoluble coatings is measured as a function of time after a water drop is placed on the surface. An initial fast decrease of the contact angle was observed which is consistent with a rapid absorption of water into the plasma polymer. A low contact angle/ high wettability correlates with coatings determined to have high carboxylic acid concentrations. By changing the ppAAc-co-Oct chemistry a change in the contact angle of 40 degrees can be achieved.