Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and 28Si n.m.r. methods. Experimental data cover the ranges: 11.36 < pH < 14.09, 2.4 < [Na+]tot/mol dmP3 < 9.6. 2.1 < [Si],,,/mol dmP3 < 9.3 with a variation of SiO,/Na,O between 3.3 and 1 .O. From available pH and n.m.r. data an equilibrium model is derived describing polymerization and protonation of silicate species in these water-glass solutions. With SiO,/Na,in the most alkaline solutions (pH 2 13), dimeric as well as cyclic forms of tri-and tetra-meric silicate species predominate. At higher SiO,/Na,O, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6 8 dominating. With SiO,/Na,O 2 3.3, i.e. close to the precipitation boundary of amorphous silica, cage-like polysilicate species dominate.The average charge per Si atom for the different polysilicate species varied between -2 in the most alkaline solutions down to -0.5 close to the precipitation boundary. The buffering capacity of the different water-glass solutions was found to decrease with increasing SiO,/Na,O.The change in the different formation constants (log p, , ) due to variation in total Si and SiO,/Na,O, was found to be linear with respect to the Na+ concentration, i.e. log p, , = log Fpq +k[Na+].Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO,/Na,O and total Si concentrations are derived using the computer program SOLGASWATER.