TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The fluoropolymer composition contains preferably 50 wt% or more of spherical fluoropolymer particles. The spherical fluoropolymer particles have an average diameter of 1- nm or more (preferably 10 - 300 nm, and most preferably 30 - 160 nm). The amount of the acid or acid-base groups contained in the surface part of the fluoropolymer particle is more than in the inner part of the fluoropolymer particle. The acid or acid-base groups bond to the fluoroether side chain represented by general formula (I).Y1 = F, Cl atom or perfluoroalkyl group ;n = 0, 1, 2 or 3 ;Y2 = F or Cl atom ;m = 1, 2, 3, 4 or 5The fluoroether side chain bonds to the C atom constituting the perfluoroethylene unit of the fluoropolymer main chain.Preferred Dispersion: The fluoropolymer dispersion contains 2 - 80 wt% of the solid fluoropolymer composition. The liquid medium is an aqueous dispersant containing 10 - 100 wt.% of water.Preferred Process: A process for producing the fluoropolymer dispersion comprises hydrolyzing -SO2X1 and/or -COZ1, which the fluoropolymer precursor contains, in an aqueous medium so as to produce the fluoropolymer having -SO3H and/or -COOH groups.X1 = halogen atom;Z1 = 1-4C alkoxy groupThe aqueous medium contained in the fluoropolymer dispersion originates from the aqueous medium in which emulsion polymerization is carried out for producing the fluoropolymer precursors. The fluoropolymer precursor is obtained by polymerizing fluorovinyl ether derivatives represented by general formula (II).A1 = -SO2X or -COZ1;X = halogen atom, -OM3 or OM41/2;M3 = alkali metal or NR9R10R11R12;M4 = alkali earth metal;R9 - R12 = H atom or 1- 4C alkyl group; andZ1 = 1-4C alkoxyl groupThe fluoropolymer precursor is a copolymer having two or more components which is obtained by polymerization of fluorovinyl ether derivatives and fluoroethylenic monomers. In (II), Y1 is preferably a trifluoromethyl group, Y2 is an F atom, n is 0 or 1, and m is 2. WIDER DISCLOSURE - A fluoropolymer dispersion comprises the invented solid fluoropolymer composition dispersed in a liquid medium. A dispersion composition for forming a thin film comprises the invented fluoropolymer dispersion and at least one alcohol selected from methanol, ethanol, propanol and tetrafluoropropanol. An activated substance fixing body comprises the invented fluoropolymer and an activated substance. An electrolyte film contains the activated substance fixing body.    EXAMPLE - CF2=CFOCF2CF2SO2F, then hexafluoropropylene and then tetrafluoroethylene were introduced into a stainless steel autoclave containing CF2=CFOCF2CF2SO2Na and persulfate ammonium aqueous solution at 0 degreesC. The autoclave was heated to 60 degreesC, and then polymerization of the mixture was carried out at 60 degreesC under a pressure of 0.7 - 0.9 MPa for 4 hours so as to obtain a fluoropolymer precursor dispersion. The resulting fluoropolymer precursor dispersion was diluted to twice its volume with water. The diluted solution was subjected to hydrolysis at 55 degreesC while adding dropwise a 10 wt.% potassium hydroxide (KOH) solution and stirring for 2 hours. A 1N hydrogen chloride (HCl) solution was added to the resulting reaction solution so as to subject the mixture to hydrolysis with the acid, then the reaction solution was subjected to purification/condensation so as to obtain a fluoropolymer dispersion. A mixture of ethanol and iso-propanol was added to the fluoropolymer dispersion so as to obtain a dispersion composition for forming a thin film. The dispersion composition was coated on a glass plate, dried, and heat-treated at 300 degreesC for 10 minutes so as to obtain a thin film (5 - 10 micrometers thick).