Stable　crystalline　CdS　nanoparticles　were　synthesized　in　Nafion　ionomer　membranes　by　using　thioacetamide　(TAA)　as　a　nonionic　precursor.　Unlike　the　ionic　precursors　such　as　Na　2　S,　TAA　could　diffuse　into　the　cationic-exchangeable　ionomer　membranes　much　more　uniformly.　This　led　to　the　formation　of　homogeneously　distributed　CdS　nanoparticles　in　the　Nafion　membranes,　which　was　confirmed　by　elemental　mapping　with　energy-dispersive　X-ray　(EDAX)　analysis.　Results　from　the　characterizations　on　the　physical　properties,　the　chemical　stability,　and　the　photocatalytic　properties　of　these　CdS　nanoparticles　embedded　in　Nafion　membranes　are　presented　and　discussed.　The　parallel　data　from　the　CdS　nanoparticles　in　Nafion　membranes　prepared　from　the　ionic　Na　2　S　precursor　are　also　shown　for　comparison.　©　2005　American　Chemical　Society.