ZE0LITES and other microporous molecular sieves(1-4) exhibit useful catalytic, absorption and ion-exchange properties. Most synthetic zeolites are made by hydrothermal methods, which generally suffer from the drawback of long crystallization times(5). Here we show that the crystallization rate of zeolites and related molecular sieves can be accelerated by a factor of as much as about five by the addition of various ’promoter’ ions, such as perchlorate, phosphate, nitrate, sulphate and carbonate. By using enhanced-sensitivity Si-29 NMR studies to monitor soluble silicate species during crystallization, we show that the promoter ions enhance the rate of formation of the oligomeric (mainly tetrameric) silicate ions responsible for nucleation and growth of the crystals.