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Abstract: A range of SiO x -like coatings was deposited on glass slides from a hexamethylsiloxane precursor by plasma-assisted CVD. The effect of varying deposition parameters, specifically ion cleaning time and HMDSO/O2 ratios, on the coating properties and antifouling performance was investigated. At low HMDSO/O2 ratios, the resulting coatings were close to SiO2. Carbon content in the bulk of the coatings increased with increasing HMDSO/O2 ratio. Coatings deposited at high HMDSO/O2 ratios and with the longest cleaning time (30min), elevated the relative carbon content to 25 atomic %. Surface energies (22–43mJ/m) were correlated with the degree of surface oxidation and hydrocarbon content. With the exception of the most polar coatings the apolar component of the surface energy (γ LW) was the dominant component. In the most hydrophilic coatings, the Lewis base component of the surface energy (γ −) was dominant. Significantly improved antifouling performance was detected with the most reduced coatings deposited using the extended ion cleaning times. For both, the removal of sporelings of the marine green alga, Ulva linza and the initial adhesion of the freshwater bacterium, Pseudomonas fluorescens, there was a strong, positive correlation between strength of attachment and ion cleaning time. Increased ion cleaning time will elevate the deposition temperature, increasing decomposition rates and thus the crosslinking of the polymer. Increased cross-linking may render these coatings less permeable to penetration and mechanical interlocking by the adhesive polymers used by these organisms, thus reducing their adhesion. Films with improved biological performance have potential for use as coatings in the control of biofouling in applications such as heat exchangers, where thin films are important for effective thermal transfer, or optical windows where transparency is important. [Copyright &y& Elsevier]