Plasma polymerization is a useful method for depositing thin films on substrates. The films formed are cross-linked, and their character depends on the plasma polymerization process conditions. The influence of power and monomer flow rate on the character of plasma-polymerized hexamethyldisiloxane (PPHMDSO) was investigated. The deposition rate increased strongly with HMDSO flow rate at low powers, indicating that the plasma is monomer-deficient. Increasing the power yielded a rapid drop in deposition rate, which reached a relatively flow rate-independent plateau at high powers. At high flow rates and low powers, the similarity between the plasma polymer with its high hydrocarbon concentration and the monomer was greatest. At high powers, the monomer was more intensively fragmented yielding a more inorganic structure rich in silicon and oxygen. Generally, the plasma polymer seems to be a silicon-oxygen network with short hydrocarbon chains that may include hydroxyl and/or carbonyl groups attached to the silicon in the backbone. The inorganic nature of the plasma polymer at high powers and low flow rates is reflected in its relatively high polar component of surface tension.