Tetraalkylammonium salts of transition-metal-substituted polyoxometalates, such as [(n-C7H15)(4)N](6)]alpha-SiW11O39Co] and [(n-C7H15)(4) N](6)[alpha-SiW11O39Mn], efficiently catalyze cyclic carbonate synthesis from carbon dioxide and epoxide. The catalytic activity is significantly influenced by the type of transition metal and the countercation (Co-2+ approximate to Mn2+ > Ni2+ > Fe3+ >> Cu2+: (n-C7H15)(4)N+ > (n-C4H9)(4)N+ >> K+). Co- or Mn-substituted catalysts required neither additional organic solvents nor additives. Thus, polyoxometalates are promising as nonhalogen anionic components of catalysts for cyclic carbonate synthesis. (c) 2005 Elsevier Inc. All rights reserved.