Solid particles and agglomerates of the particles were found to be produced in octafluorocyclobutane (C-C4F8) plasmas used for reactive ion etching Of SO2 and chen-deal vapor deposition of low dielectric constant fluorocarbon films. Analyses of the solid products elucidated a part of the chemical reactions related to the polymerization process in gas phase. The particle production depended strongly on the feed gas pressure in the plasma. A marked increase of the production was observed as the pressure became higher than 50 mTorr. The production accompanied the pressure depression at the steady state in the discharge, indicating the depletion of the gas species owing to polymerization. Furthermore, the partial pressure of stable molecules such as CF4 and C2F6 produced in the plasma and the film deposition rate on surfaces changed drastically at the critical pressure. These pressure dependencies suggest that polymerization reactions between reactive species and source molecules in gas phase lead to the particle production.