A process for the preparation of plexifilamentary film-fibril strands of synthetic fiber-forming polymer comprises the steps of generating a spin fluid and flash-spinning the spin fluid to form plexifilamentary film-fibril strands of the synthetic fiber-forming polymer. The spin fluid consists essentially of (a) 5 to 30 wt. % of a synthetic fiber-forming polymer, (b) a primary spin agent selected from the group consisting of hydrocarbons with 4 to 7 carbon atoms, and chlorinated, brominated and fluorinated compounds, and (c) a co-spin agent co-spin agent selected from the group consisting essentially of fluorinated organic compounds containing 4 to 8 carbon atoms and a double bond, and having an atmospheric boiling point of less than 100.degree. C. The co-spin agent is present in the spin fluid in an amount sufficient to raise the cloud point pressure of the spin fluid by at least 50 pounds per square inch (345 kPa). A spin fluid used in the process of the invention is also provided.