Siloxane materials are polymeric materials which have a main chain constituted of siloxane bonds with various functional groups as side chains. They have important applications such as electrical insulation, heat resistance, chemical resistance, weatherability, and are expected to be used in the fields of optical, medical, electronics, and food applications. To improve and design the functionalities of polysiloxanes, it is important to understand the relationship between molecular structure and properties of siloxane materials. Siloxane polymers are mainly synthesized via the hydrolysis and condensation of silicon alkoxide via the sol-gel method. In this process, the alkoxy groups are hydrolyzed to form hydroxyl groups and subsequently condense to form an oxo bond. However, it is difficult to control precisely the molecular structure of the resultant materials because of a competitive contribution of hydrolysis and condensation on the molecular structure of products, occasionally resulting in less reproduceable physical/chemical properties. In this study, we focused on silanol groups as a key moiety to understand the relationship between the molecular structure and physical properties in the siloxane oligomers. The residual silanol groups are known to affect the properties of the products such as hydrophilicity, refractive index, permittivity and long-term stability of siloxane polymers. Accessibility and thermal stability of silanol groups could be positively controlled for the rational structural design of siloxane networks. To this end, we synthesized siloxane oligomers prepared from PhSi(OMe)₃ (Ph=C₆H₅, Me=CH₃) and Me₂Si(OMe)₂, and evaluated the thermal stability of the silanol groups as a function of heat treatment. A difference of the thermal stability of silanol groups on Me₂-Si and Ph-Si units was investigated by ²⁹Si nuclear magnetic resonance (NMR) spectroscopy after the heat treatment. The thermal stability of silanol groups varied depending on the type of functional group attached to the silicon atoms. Siloxane films synthesized from oligomers were heat-treated at different temperatures to control the amounts of silanol groups. The surface wettability of the films was investigated to understand how the amounts of silanol groups could affect the surface property of the resultant siloxane materials. The surface wettability of the siloxane films prepared in this study was not depending on the amount of silanol groups. This is presumably because the access of water molecules to the silanol groups was prevented by the steric hindrance, suggesting that the control of steric hindrance is important in siloxane material design.