We have investigated the thermal annealing effects of hydrocarbon species on the methane-plasma exposed silicon surface, investigated by in situ infrared absorption spectroscopy (IRAs) in multiple internal reflection (MIR) geometry. The proportion of types of hydrocarbon species is not remarkably changed in the hydrocarbon network that consists of sp-, sp(2)-and sp(3)-CHX species by annealing at moderate temperatures. On the other hands, the proportion is drastically changed in the network that is mainly composed of sp(3)-CHX species by annealing at moderate temperatures. It suggests that excess CH3 species in the film is not stable against thermal annealing and would be converted to sp(3)-CH1-2 species by annealing at moderate temperatures. And the data also show that sp(2)-CH species is more stable against the thermal annealing than sp(3)-hydrocarbon species. (c) 2005 Elsevier B.V. All rights reserved.